You must store enough energy in it to send the right units and cast the right spells. Your goal is to destroy the enemy tower before the end of each timed turn. You can try to destroy all three towers, but if you only cause more damage than your opponent, you have won every battle. Use your orcs and imps to defend your tower and send them across the bridge into enemy territory. Try to get your orcs close enough to attack the tower, and use remote magic spells to support your orc attacks.