Brother Martin and Kvatch
There are two ways that you can approach this. Either you have spoken to Jauffre at Weynon Priory or you just came to Kvatch on your own. There is little to no difference in what you are going to do, but the result of your actions depends on how you got here. Kvatch is also divided into three separate stages. The first stage is rescuing the civilians from the Chapel, the second stage is clearing the town and getting into the castle and the third stage is clearing the castle. Only the first stage ("Breaking the Siege of Kvatch") needs to be completed for the Main Quest. The second and third ("The Battle for Castle Kvatch") is optional and is more properly a Miscellaneous Quest or possibly part of the "Allies for Bruma" quest a bit farther down the line. I have included it here because it's easier to get it out of the way here than to remember to do it later.
You don't actually fast-travel to Kvatch itself. Instead, you arrive at a sort of refugee camp below the city. One of the first things that should happen is that you'll be approached by one of the NPCs, who will initiate a conversation with you whose purpose is to let you know what happened at Kvatch (the city was invaded and destroyed by Daedra). I've run into a couple of instances where he did not appear at all, so many of the NPCs in the came lacked appropriate conversation topics. Short version: city destroyed, guards holding the line at the top of the hill, it's the end of the world, aaaaaarrrrrgggh!
If you have begun the Main Quest, you'll find out that Martin is not in the camp (like you were actually expecting him to be?). There are a couple of nice things about the camp, though. First off, there are beds there. Secondly, you have a Smith, an Alchemist and a general merchant available to you. The Smith has no goods available, but she'll buy and repair your stuff. She's also a Master of Mercantile, so you aren't going to be getting much for whatever it is that you sell to her. But at least she can fix stuff and give you something for your loot. The Alchemist has a small selection of potions, but will buy any potions or ingredients that you come across. She doesn't have much money, but if you sell things one at time, you'll still be able to get a decent return on your efforts. The same for the general merchant - no goods, but willing to buy.
So hoof it up the hill until you get to the barricade at the top. See that big fiery thing on the other side? That's an Oblivion Gate. You're probably going to be seeing a lot of those in the coming days/weeks. Play your cards right and you can learn to say, "phat l00t" with the best of 'em.
Savlian Matius is the guy in charge. At first he tries to chase you off, but if you tell him that you're there to help, he'll tell you to go through the gate and find a way to close it. So do just that. You'll run into three leveled Daedra when you approach the gate. The guards should come out from behind the barricade to help clear the way for you as soon as the critters get within their detection range. You don't actually have to fight them. You just have to get close enough to the gate to activate it. Activate the gate for your first trip to Oblivion.
On the other side of the gate, you're at the top of a hill looking down to a HUGE gate at the end of a bridge. You'll see two leveled Daedra (probably Scamps or Clannfear) come running across the bridge and a Kvatch soldier fighting a third one off to the left. You should help the guard by taking out as many Daedra as you can. Once all three are dead, you can either send the guard back to Kvatch or have him travel with you. I've always sent him back so that I can have more cannon fodder later. But if you have a way of healing a companion (which I seldom do at this point), you can take him along.
Now that everything has quieted down a bit, this would be a good time to look around and get your bearings. You cannot open the big gate from this side (natch). Around the area, you'll find three plants that you can harvest for ingedients: Bloodgrass, Harada and Spidal Stick. Bloodgrass is just Oblivion's version of grass. It won't do anything to you in and of itself. The Spidal Stick plants, if you get close enough to them, will release a cloud of poison spores that will temporarily drain your speed. Some of the Harada plants will try to hit you (the rest just kind of hang there). I find them more of an annoyance than anything else, but they can push you out from whatever cover you might have near them and they do put a minor hurt on you (5 points) each time they hit.
Almost without exception, all creatures in Oblivion are leveled creatures. So, depending on your level, you can run the full range from Stunted Scamps (the weakest) to Clannfear, to Atronachs, to Spider Daedra and Xivilai (the strongest). In addition, you'll run into Dremora of all levels and all sorts - melee types, missile types, spell types. All of them will give up at least a weapon (personally, I find them to be too heavy to be worth the effort of toting them back to sell) and a Daedra Heart. Some carry scrolls, potions and enchanted weapons. Not always, but often enough that anything wearing a robe should be a target of choice. If your level is in the teens, you can get rich just off of Dremora and the stuff they carry.
Now that you've had a chance to soak in the situation (not good), let's get on with it. Your goal is the top of the big tower - remember that for any other Gates that you might go into. Work your way around until you're near a second big gate. The tower behind it is your goal for the present. When you get to the tilted bridge, wait. Rocks will start rolling down the hill and across the bridge. If you're lucky, they'll take out the Daedra that wanders onto the bridge. If you're not lucky, you'll have to take it out yourself. Just before you get to the bridge, you'll see a hanging pod-looking thing. These are the Oblivion equivalent of chests. Their contents are random, so perhaps you'll get something good, perhaps not. Just hug the rocks on your right (watch out for Harada Roots) and take care of any Daedra that you come across. There will be one up on the rocks above you and another on top of the rocks directly ahead of you, but the rest are all on the ground. Eventually you will get to the tower behind that big gate. Enter and deal with the Daedra on the ground floor. That thing with the spikes sticking up (the "Corpse Masher") is an elevator of sorts. Step on it and walk to the center and it will start to rise, taking you up to the second level. A spiraling ramp goes up from there and you'll find another Daedra at the top. You'll also find a lever that opens a gate. It's not the one outside; it's the one that you saw when you first came through the Oblivion Gate.
OK, go back the way you came (watch out for the spikes on the Corpse Masher on the way down). You'll have to deal with more Daedra, but get back to where you're kind of catty-corner from the big tower. See those orange spiky things on the ground? They are the Oblivion equivalent of land mines. They'll spring up and explode if you get too close to them. You can trigger them from a distance with a well-placed arrow, but they'll occasionally shoot a fireball back at you. In front of the big tower, depending on your level, is either a live or a dead Dremora. He has Dremora Field Arrows on him. They aren't worth much back in Cyrodiil, but they pack a decent punch and can be recovered. They also weigh 1 pound each. Your call on whether to pick them up or not.
If you follow the road down from the tower, you'll get to the bridge with a couple of Daedra on it. Take care of business. You'll also find the bodies of four of the six soldiers that Matius sent. The last soldier is in one of the smaller towers near the big one and we'll get to him later. You can loot their bodies if you wish. The only item of real interest is a Dagger of Numbing (5 pts Frost Damage on strike), but now that the gate is open, you can haul it back and sell their stuff to the smith. If you're low-level, their chainmail may be better than the armor that you currently have, though, and their weapons are Steel.
Once you done whatever you're going to do, go back up the hill and into the big tower. The center of the tower is known as the Blood Feast. The outer ring that goes between floors is called the Corridors of Dark Salvation. There are a couple or three Daedra on the ground floor of the Blood Feast. Go through one of the doors on the side into the Rending Halls. Work your way up to the next floor of the tower and go back into the Blood Feast. Go up the ramp and into the Corridors of Dark Salvation and deal with any Daedra or Dremora that get in your way. From here, two of the Citadel Doors are locked, so there is only one way you can go - out to the Plane of Oblivion. This is a bridge between the big tower and one of the smaller towers. You're probably out of the detection range of any of the Daedra that might still be wandering on the ground, but this will not always be the case when you go through other gates. As soon as you enter the tower, you'll hear someone yelling at you. That's the last soldier. Just follow the ramp upward, take out the Sigil Keeper and get his key. Even if you have a high Stealth skill, you can't talk to the prisoner without unstealthing, so don't bother trying to sneak by the Dremora to do it. You can't rescue the soldier (he tells you so if you talk to him a second time), so just take the key and head back to the main tower.
Now that you have the key, you can unlock the two other doors. One of them will take you up to a dead-end floor in the Blood Feast, but there are a couple of beating-heart things called "The Punished" which may have some decent loot in them. The other door will let you go higher in the tower. Just keep working your way up. Once you're back in the Blood Feast, follow the ramp up to a floating platform. Activate it to be teleported up to the next floor. Follow the ramp up until you get to the Sigilum Sanguis. The doors are locked, but you have the key. Even if you somehow managed to drop the key somewhere along the way (not likely since it's a quest object), these doors can be picked. Which door you use doesn't really matter. They both go to the same place. In the central chamber, you'll see bone stairs leading up to the next floor. It doesn't matter which set you use. You'll find two more Punished on this floor, plus one or two Dremora. You'll also see ramps up to the red membrane-looking platform above you. Go up and deal with the last Dremora, loot the last container and activate the Sigil Stone.
A Sigil Stone will be added to your inventory (what kind depends on your level). You can use these stones to create enchanted items. Each stone carries two possible enchantments. One will enchant a weapon and usually carries a Destruction-based spell effect. The other is intended for a piece of armor, jewelry, or article of clothing and carries a constant-effect booster or protection spell of some sort. The higher your level, the bigger the enchantments will be.
Once you have activated the Sigil Stone, you'll be returned to Kvatch and the Oblivion Gate will be closed. It's a decent explosion, too. One time, I had managed to kill a Fire Atronach that fell down in the Gate itself. When I closed the gate and was headed back to Matius' position, I saw the body falling WAY off in the distance. Wonder if there might be an equivalent to Halo's "Warthog Jumping" here? But I digress.
Talk to Matius again (either before or after hoofing down the hill to unload your loot) and he'll suggest an attack on the city gate. When you're ready, let him know. He and the guards will charge the gate and enter Kvatch.
In the area just inside the gate you'll encounter some more leveled Daedra. Matius can't be killed, but his soldiers can be, so do what you can to keep them alive until the immediate area has been cleared of Daedra. Then talk to Matius again. He'll tell you that it's now safe to get the civilians out of the Chapel, so go inside with him. The civilians will file out and head down to the refugee camp. If you have talked to Jauffre, you should be able to pick up Martin now. If you haven't, he'll head down to the refugee camp and wander around until you do go talk to Jauffre or until you're ready to pick him up.
As far as the Main Quest is concerned, you're done with Kvatch and the "Breaking the Siege of Kvatch" quest in your journal is completed. When you talk to Matius again, you'll get the final journal update for "Breaking the Siege of Kvatch" and you'll get a new quest, "The Battle for Castle Kvatch". Your job is to help Matius clear out the rest of the town and the castle. It's not critical that you do it now, but you should take care of it before you finish the "Allies for Bruma" quest.
Once you're ready to go, talk to Matius again and tell him so.
Matius and any surviving soldiers will charge out the other side of the Chapel and into the town. You'll find a few more leveled Daedra out in this area. Help the soldiers as best you can. Matius is still "essential" and can't be killed, but the solders can be. If your combat skills are good, just get out front, stay there and take care of business. If your combat skills aren't so hot, but you have targeted healing spells like Convalescence, stay back and play healer on those soldiers. Otherwise, do the best you can.
I'm not too fond of this part of the quest because it relies on the game's AI to handle the NPCs and they have a nasty habit of just jumping in on whatever is handy, which means they really get in my way. Since most of the Daedra will probably have a ranged spell of some sort (Scamps and Flame Atronachs, for example), they'll typically stand back until they get off a couple of spells. This makes them decent targets if you can get out in front of the rest of the soldiers. When the handful of Daedra are finished, Matius and crew will try to storm the castle, but the portcullis is down. There are a couple of respawning Daedra on the roof of the castle who will be hitting Matius and crew with ranged spells. If you're a good shot with a bow or have some decent ranged spells yourself, you can take them out for a little while. Otherwise, you'll just have to do the next part quickly.
Matius sends you to get the gatehouse key from Berich Inian, who is still in the Chapel. It doesn't matter whether you're nice to him or not, he's not going to give you the key. Instead, he's going to lead you through the Chapel Undercroft into the east section of the city where there is a passageway that will take you from the Guardhouse to the Gatehouse, where you can open the portcullis. He tells you to be sure and get the key off of his body if he does't make it (you'll get a journal entry to that effect if he doesn't). You'll also pick up three Imperial Legion soldiers to help you out (two melee and one archer). Follow the same tactics that you did earlier: either get out in front and stay there, hang back and provide healing or do whatever you can to help. All three Legionaires are set to ignore friendly hits (so they won't turn and whomp on you if you hit them by mistake), but you can still pick up an assault bounty or worse because they're part of the Legion. It's not terribly critical that any of these guys survive, but there are about 20 leveled Daedra to deal with in this part (three or four are in the Undercroft and the rest are in the eastern section of the city). And any survivors will help with clearing out the castle itself. Considering the NPC combat AI, this may or may not be a good thing.
Once you reach the guardhouse, you can breathe a little easier (well, except for all of the smoke). Activate the hatch and just cross to the ladder on the other side. Nothing to fight in here (ain't that nice). When you come back out in the Gatehouse, run over to the other side and activate the wheel that raises the portcullis. Any surviving soldiers who are with you will join those waiting outside and charge into the castle courtyard. There are eight or so leveled Daedra in the courtyard. When they're dead, Matius will talk to you and then you can charge into the castle itself. There are five leveled Daedra in the first chamber of the castle.
After the first chamber is clear, Matius and his crew will hold their position and send you on to rescue the Count. You're not completely on your own, though. Any of the three legion soldiers who have survived will continue to follow you, as will either or both of the two Kvatch archers if they are still alive. You could potentially have as many as five helpers or as few as none for the next section, depending on how good you were at keeping them alive. There are four leveled Daedra in the dining room area. Although there are a couple of rooms off to the side, there isn't anything of importance in them. Your goal is directly ahead: the Count's quarters.
In the Count's quarters is one leveled Daedra. Take it out however seems most prudent to you. You're too late to save the count, though. Activate his body to get a journal update about retrieving the Count's signet ring. Sure enough, there's a "Colovian Signet Ring" in his inventory. Take it (and anything else in the area - it's not like the Count is going to be complaining). Now return to Matius. You have a choice to make in your dialog with him. You can either let him down hard or let him down easy as far as reporting the news of the Count's death. If you let him down hard, his disposition will drop 20 points (which may hurt you later). If you let him down easy, his disposition will rise by 10 points. Give him the ring and he'll give you his cuirass. It's light armor and has a leveled Fortify Stregth and and Endurance enchantment on it. If you've handled this quest well, Matius will send a soldier to fight with you in the "Allies for Bruma" quest.
Now that the quest is completed, go ahead and pick up any loot from the area that you might want. There is a skill book, "A Dance in Fire, vol. 4," immediately to your right in the second chamber. The rest is cheap-o stuff, but there's a lot of it. It won't net you much gold, but it will probably get you a couple of points to your Mercantile skill when you sell it due to the sheer quantity. Check your stats journal to see whether or not you've picked up a bounty for "friendly fire". If so, you'll need to take care of it before you try to talk to any Imperial Legion soldier. If not, you're done.