Battle Horrors and Doom Guards
Battle Horrors and Doom Guards are two of the hardest enemies encountered in Durlag's Tower. These are heavily armored enemies that can deal large amounts of damage in melee combat. It is usually best to avoid melee combat against both of these enemies for this reason. Use the Monster Summoning or Animate Dead spells to create temporary allies. Send these allies rushing toward the Battle Horrors or Doom Guards. Stay back and cast damage spells or fire missile weapons to kill Doom Guards. Battle Horrors are immune to missile weapons, so you can only use damage spells against them. You should have little trouble killing either enemy before it kills off all of your temporary allies. The Demonknight in the throne room is effectively a stronger Doom Guard. The same tactic can be used against him.
Traps
You need a party member with the Thief class before attempting to enter Durlag's Tower. Virtually every treasure chest and passageway in the tower is trapped. Even if each individual trap is not particularly dangerous to your party members, the cumulative effect of so many traps will eventually consume the party's healing spells and items. Move slowly through the tower while the Thief uses the Detect Traps skill to locate each trap. Once located, have the Thief disarm the trap before proceeding further.
Sparring Room Puzzle
The sparring room on the second floor of Durlag's Tower has a puzzle that must be completed before you can enter the throne room on the same floor. You need to pick up the three rune stones from the bed room. The rune stones are orange, red and green. The dummies in the sparring room are each a different color. Place each rune stone into the same colored dummy in the sparring room. Attack the central dummy after the rune stones are in place. A message appears after a few weapon strikes to indicate the passageway to the throne room is not clear.
Kirinhale
A succubus named Kirinhale can be found at the top of Durlag's Tower. She was cursed by Durlag to remain within his tower for all eternity. You have two options for dealing with her. You can offer to find a human to take her place. This option is only possible if you did not kill Riggilo, a thief whom you encounter earlier in the tower. She gives you a lock of hair after you offer to help her. Go back to Riggilo and give him the lock of hair by telling him it is from a nymph. This causes Riggilo to be trapped in place of Kirinhale. Return to Kirinhale after doing this to gain 4,000 experience points. Riggilo attacks you the next time you talk to him. Alternatively, you can fight and kill her. She is a potent magic user, but not very difficult to defeat. Her only major ability is Dire Charm, which can cause one of your party members to temporarily aid her in combat. Use Dispel Magic to break this effect.
Demonknight
The Demonknight serves as the final enemy of the underground levels of Durlag's Tower. There are two different ways to approach this fight. The Demonknight is a much more powerful version of a Doom Guard. If you can prevent him from getting close to your party using magically summoned allies, you can kill him with spells and missile weapons. However, he is stronger than a Doom Guard, making him much more capable of fighting his way through your allies. You can also attack the mirror he is standing on. Attacking this mirror causes it to summon a large group of creatures that are hostile to both your party and the Demonknight. If you keep your party back, the creatures almost always kill the Demonknight without assistance. You then are left with killing the remaining, usually wounded, creatures that finished off the Demonknight.