From The Heroes of Might and Magic III wiki

Class	Earth Magic
Basic cost	20
Duration	Instant
Level	3rd

Does one point of damage to random castle Walls and/or Arrow Towers.

Level	Effect
None	Damages 2 walls
Basic	Cost reduced by 3
Advanced	Damages 3 walls
Expert	Damages 4 walls

[edit] Comments

Let's admit it. When invading a town arrow towers are annoying, and this spell makes your job much easier. Since the maximum amount of times a catapult can fire is two, you'll probably need some back-up to help out in bringing the walls down faster. If one hasn't found a Stronghold yet this can come in handy instead. It's steep mana cost is one of the only sad things about this spell.