Traps
Traps such as Beamos, Blade Traps, and Thwomps can be built just like Monsters.
Traps can only activate under certain circumstances, often having an Instinctive intelligence – their tactics are set in stone.
Traps suffer half the Evasion penalty for their Defense. Traps also never use up their MP and SP. Unlike creatures, they never tire. However, they must still have enough MP or SP to use their Spells or Techniques.
Typically, Traps are defeated not by beating them into submission (like Monsters), but by deactivating them. Consider how Heroes might stop your Traps from firing – be it temporarily or permanently – and incorporate those mechanisms into your encounters!