One painting has a VERY good life-sized portrait of some fellow. After the PCs pass by, a trompe l'oil steps out of the painting. This monster's job is to slay anybody who invades the tower. It prefers to launch attacks from behind; in 3D form it can use any mundane gear, such as a bow-and-arrow, pull the lever to trigger a trap, &c. It knows all about the other tricks and traps within the tower, and will re-set them to be ready for the PCs. (The PCs can kill it by the usual HP-removing methods, or by destroying its painting.) Inspired by something I saw in Pathfinder Horror.
Presumably the evil cult has enemies, rivals, and opponents. Devils and demons, for instance, do not get along well together. Summon one to guard an item coveted by the other.
Put an item in a "summoning circle" - a Circle of Protection against Evil, which is set to keep inside things in. The REAL trap is the Glyph of Warding written under the item, which sets off a spell instead of doing damage. What would be extremely difficult to deal with when one member of your party is stuck in place? (upcast Fireball or Phantasmal Force comes to mind; so does Storm of Vengeance if the Wiz was high-level.)
Perma-cast Guards and Wards on the castle hallways. This doesn't do anything by itself, but if the party is constantly getting lost, they will take longer before finding a safe place to rest &c.
Explosive overcoat of a vest that looks for all the world like Padded Breastplate Armor. The explosives are triggered by body heat and sweaty dampness (body salt is a necessary component of the reaction).
Classic "looks like a trap to me" but some greedy fool will still fall for it: Obviously fake book sits on table, with several valuable gems inside the 'hidden compartment'. The gems are cursed:
- bearer attracts unwanted attention, wandering monsters, &c.
- similar to 4e Warlock's curse, bearer takes an extra 1d4 from the first source of damage following his turn in every combat round
- gem is actually a semi-precious gem that has been shined up to look more valuable than it is. (Quartz = fake diamond, &c.) The bearer doesn't see this (individual-specific illusion) but the people to whom he tries to sell the gem can see it clearly. They will be unhappy with him for trying to rip them off, and might attack outright or might make his name Mud in the local society.
- Gem is a magical bomb and explodes for say 1d6 per 10GP estimated value of the gem after being picked up. (Scale the damage to tempt the party healer into action, not to insta-kill somebody.)
- gems have contact poison all over them; must wear gloves when you pick them up to avoid effect.
One room is artificially warmed. So much so that the PCs will take exhaustion levels if they stay too long. THEN something bad happens to them. (White Plume Mountain, in the Yawning Portal AP, has a hallway trap that you can use as a model.)
Highly-magic room has walls of fire, ceiling of fire, &c. If you end your turn next to one, you take small-ish amounts of damage. This room doesn't have any other traps in it, it's a place to build up atmosphere.
...or maybe the Room of Flame has a classic Grimtooth Trap in it: the door is ajar opening into the room and a bucket sits atop it. The bucket is full of water, spelled to not evaporate in the heat. When the PCs walk through the doorway, that water will be HOT - maybe even boiling. Splash trap attacks everybody in the square (or two) below it when it topples.
After the party figures out they need Protection from Fire, have an enemy who is Cold-infused who does extra damage against targets who have Protection from Fire spells on themselves.
Portal to the Plane of Fire. This is the shortest way to proceed deeper into the tower.
Portal to a part of the Plane of Earth that forces you to cross above a hot springs, mud pit, geysers, &c (Yellowstone National Park for inspiration) to find something important.
