Luna placed a shield of her own over the entrance to the middle layer of the city, holding back the soldiers running up the magical ramp long enough to get everypony away, then ran herself, dropping the shield and firing a blast at the stockpile of gunpowder around the cannons. The explosion set off another after another that tore through the ponies running up the shield, and the shield itself. Luna could hear the screams of ponies as they fell. More importantly, the blast had sent the cannons flying off the edge of the city, denying them to the enemy.