It's a really good game, with two deficits: 1. Glitches. Sometimes Monsters Freeze. That is not the end of the world, it just adds difficulty. However when The Heroes become invincible and my monsters don't attack them, that's a crippling bug. You need to add something to reset the Heroes when that happens. Maybe make that a feature? Heal the hero completely but make him attackable again.
The second thing is more speeds, or making the sped up speed faster.