Some suggestions to include new elements for strategy:
Ladders : defend the wall of a castle by dropping rocks or boiling oil on the enemies climbing ladders,
Drawbridges,
'Rain of fire' firefighters,
Enemies or your own troops using flares to divert missiles,
Lighthouse towers for caves or night stages,
Sorcerers capable of creating/destroying roads by parting water from seas,lakes,rivers,etc,
Vortex that pulls your enemies to its center,
Trenches to slow enemies down (see "Trench Warfare"),
Selective attack (see "Spirit Bomb" and "Voodoo Hut"),
...