Underworld Overlord

Underworld Overlord is a tower defense game set in the fantasy universe of the Ultima Underworld series. As the titular overlord of the Stygian Abyss, a massive underground dungeon, the player is tasked with repelling waves of invading heroes who will stop at nothing to destroy the overlord's animus, the source of their power, in each level. To defend their animus, the player must place allied monsters and traps at specific points around the level which will automatically attack any enemies that come within range. The player can also participate in the battle directly by casting spells to aid their minions or hinder their enemies. New spells and minions are unlocked by successfully repelling waves of heroes.