Tidal islands are islands that are connected to a mainland but this connection is transient and subject to the tides. During low tide, a sandbar or tidal flats may be exposed, connecting the island to the mainland. During hightide, the water level increases, and the island becomes a true island, separated from the mainland. In fictional worldbuilding, tidal islands may be holy places, or may be sites of defensive buildings such as castles or watchtowers. The tidal waters that separate the island may be treacherous.