Planktonamor's tower is Planktonamor's place or the "Evil lair" in the episode Dunces and Dragons.
The ironic thing is that Planktonamor gets defeated here when SpongeBob and Patrick rescue Princess Pearl from his grasp.
See the main article Dunces and Dragons for more information.
The tower is a dark, lifeless land surrounded by stalagmites (sharp, pointy rocks) jutting up from the ground. The tower itself is very tall with a staircase spiraling through the middle: