Taking the Keep:
Take note that the Lord of each Keep is vulnerable to piercing and freezing weapons, so have these weapons at hand!
Also, the only rooms you are able to recall from is the room before entering the portal to the Lord, and the Lord's room. Once in the Lord's room, there are no exits other than to cast the spell of 'word of recall' or to recall.
If there are no defenders present, then all should enter into the portal to engage the Lord. Those that are able to cast should begin placing maladictions upon the Lord. The maladictions should go in the order of weaken, plague, then poison. Once these three maladictions are on, healers should save their reserve energy to heal either themselves or the group members. Damage spells are also advised, but healing should take precendence before that. Those that are able to stun can attempt to do so too. Continue to engage the Lord and he will fall with time.
With defenders present, it would take a few preparations before entering the room of the Lord. Those that are able to cast should enter first and place maladictions upon the Lord while those better with their swords should remain outside in case defenders come. When a person defending does come, they will arrive in the room with the portal leading to the Lord. The ones able to hit should engage them promptly to prevent them from jumping into the portal and attacking the casters. The only door is the one to the southeast, so close it to prevent the defenders from fleeing, or open it if you need to flee. If they come in numbers, a few tornadoes can possibly even the odds.
These are only the basic tactics, so you might need to improvise given each situation!
You will return to the Altar room with nothing lost should you die fighting the Lord, so don't worry about that.
Taking Citadel Keep:
This Keep is a bit more complicated than that of Thorn and Misery. This Keep is much bigger and consists of three separate parts. Also, it should not be attempted with only a few people!
Before going into the portal, protect yourself against those of golden aura because you will be fighting against a lot of them in order to reach the Lord.
Once you enter the portal, head all the way north and clear the guards there if there are any. To the west, there should be a group of five guards. One of those should hold a key which cannot be dropped once a person picks it up from the corpse. Obtain this key and have that person run to the east where there will be a door to open. Be sure that you are not flying and that you remain visible before entering into the room opened by the key! While that single person is doing this, the entire group should head all west and all north. Once the single person steps on the plate, it should open a door for the entire group, go into the newly opened room. There is a lever to pull which keeps the door open for that single person to go through and regroup.
Once that person rejoins the group, head east and there should be another group of five guards. Repeat the process above, but this time the person with the key should walk all east and then all south for the door while the group waits for the door to open to the south. Again, pull the lever and wait for the single person to join. Search out another group of five or so guards and pick up the key again. This time, the door will be directly north. Open it and it will lead to a tower with floorplans exactly the same as Misery and Thorn. The directions again are, up, southeast, southwest, northwest, up, southeast, which leads to the safe room. The idea in taking this Lord is the same as the previous two Keeps. This requires changing your protection to that of people with no auras before combating the Lord! Do not forget about healing and preparing fully before trying to take on the Lord.