Shown at the end of this video , barrels contain explosives that can destroy even the best castles.
Barrels are used by Bricktrons to collect crystals. They can be moved to any location. Barrels are created with a piece of log, and can be used as ammunition for catapults once the barrel is 100% filled with crystals. A barrel can hold 10 crystals before full. When the barrel contain enough crystals, you can click on it and make it explode even when it's not 100 % filled with crystals.