Defense of Bruma
This is the big show-down that you've been waiting for (sort of). All of the allies that you have managed to gather are camped outside of the castle in Bruma. You might think that the object is to protect Bruma, but it isn't. The object is to get the daedra to attack so that Martin can get his hands on a Great Sigil Stone, which is the last component that he needs to open a portal to Cameron's Paradise.
After you finish talking to Martin, he heads off for Bruma along with Jauffre (and Baurus, if he's still alive). If you walk along with him, Martin will explain what's going to happen and why it has to happen that way. Your primary job, though, is to arrange a meeting between Martin and Countess Carvain at the Chapel of Talos. So do that and watch the exchange. If you have not completed all of the "Allies for Bruma" quests, you'll have the option of doing more recruiting. If "Allies for Bruma" is completely done, then your only options are to say that you're not ready or "Let the battle begin" (the signal start the whole thing rolling). This will be your last chance to tie up any loose ends, gain levels, or whatever, so take advantage of it. Martin, the Countess and all of the rest of the crew will just hang around in the Chapel until you give the go-ahead.
Once you've kicked things off, everyone marches out and through the front gate to the cheers of the townspeople. Once there, Martin will give a little pep talk to the soldiers and then the smaller Oblivion Gates will start opening. Your job in this whole mess (and it's really a mess) is to keep Martin alive and safe until the Great Gate opens. You'd be well advised to keep Jauffre and Baurus alive, too, since they'll be able to help you in the final fight. Baurus, if you'll recall, stopped being essential back in the sewers of the "Path of Dawn" quest; Jauffre stops being essential here. But keeping Martin hearty and hale is your main mission. Once the Great Gate opens and Martin is safely disengaged, you can go do what you need to do without worrying that he'll get whacked in your absence.
You'll have to fight off several waves of leveled daedra as each of the smaller gates opens. The best tactic I've come up with is to just plant myself in front of each gate and beat on anything as it spawns. I generally take a helluva lot of damage by doing it that way, but critters (daedra included) will generally engage the last thing that did them damage, so if I'm the one damaging, they're laying off of Martin. If you're running a lower-end machine, expect a completely crappy framerate during this sequence, which can make things even more difficult. Martin does have Restoration magic, so he can heal himself to a certain extent. You'll probably want to stock up on either spells or scrolls that can do ranged healing as a backup.
Eventually the Great Gate will open and your job is going to be to retrieve the Great Sigil Stone. Check to make sure that Martin is safely disengaged before you transition or he might die in your absence. When you're ready, activate the Great Gate and get ready to run like hell.