From The Heroes of Might and Magic III wiki

Class	Water Magic
Basic cost	7
Duration	Instant
Level	2nd

Can remove objects from the battlefield

Level	Effect
None	Only non-magic, non-integrated objects can be removed
Basic	Cost reduced by 2
Advanced	Fire Wall can be removed
Expert	All non-integrated objects can be removed

The following objects counts as integrated:

• Castle Walls
• Cliffs
• Rifts