In Siege Island, players take command of the defense of several castles, fending off waves of adversaries that assail from both air and sea. To accomplish this, they must erect protective walls around neutral castles, asserting dominion over the territory inside.
Once secured, they can position cannons and ballistas within the controlled area to repel enemy attacks during the attack phase. Afterward, players must reconstruct the walls using Tetris-like sections during the building phase.