If someone, anyone, casts the Ceasefire spell you cannot directly attack your enemy.
During a recent Ceasefire, I was bored and used my Titan to attack an elephant standing next to an enemy wall. He damaged the wall, and cuz elephants take several hits to kill, he destroyed the wall. I moved my troops inside, right next to the city center, and waited for the timer to run out.
Same city that had cast Ceasefire, actually.