Tunnels and the armed peasants that emerge from them are one of the biggest threats to your catapults. If peasants emerge behind your catapults and your troops advance too far ahead of your catapults it leaves them wide open. A large group of peasants can destroy dozens of catapults very quickly if they come close. It’s pretty easy to determine where a tunnel is in castle by just selecting a 5 x 5 formation of troops like pictured on the right. Move the group of troops around until one of them turns blue. This is the location of the tunnel.