Invasion

The map is impassable for the hero. Heroes with armies three times larger than mine keep appearing. I easily break through the garrison, and they attack the castle. Why are there so few units in the joining stacks? Especially those that are guarded. Three knights guard 60 thunderbirds, so what's the point of taking them? I thought I'd level up Darkness and carry through with Vampire, Berserk, and Subdue, but the magic school only goes up to level 3... I think at least one of these spells should be included. Respect to anyone who can complete the game as a hero. I'll try starting as a hero again, but...