All castles in Heroes 3.

Okay, the spell is fine. The terrain can either greatly help or hinder in battle, and the outcome often depends on it (for example, when a trench runs across the entire screen, you can only bypass it from above or below). But in your case, you have elves at the top, and they are blocked by, for example, dwarves! Try to approach them, but if the map doesn't have this trench, units can approach you from all directions, and...
In general, I always protect the elves with unicorns, dwarves, and ents, while the centaurs and pegasi stand a little lower and wait for suitable troops!