Auto-translated
"Earthquake" became overpowered when used as a hero skill. And, by the way, it's unclear why it was removed as a spell in the summoning school.
The damage it now deals is disproportionate to the damage from towers, as experienced by the attacking side. A catapult with this skill is now often unnecessary, as the defending side is better off moving everyone outside the castle walls, including archers, and fighting in the open.
I understand that you might have made this skill available through Sulfur Rain, forcing a difficult choice between upgrading the ballista and this overpowered skill. And even then, the walls would still be destroyed. But no, this skill is extremely easy to acquire, doesn't require sacrificing anything important, and can be obtained at early levels.