Portland Castle is one of the Device Forts, also known as Henrician Castles, built in 1539 by Henry VIII on the Isle of Portland to guard the natural Portland anchorage known as the Portland Roads. The castle lies at the far north of the island, on the edge of the village Castletown, which was directly named as such from the castle. It was strategically positioned next to the tidal lagoon The Mere, which was later completely filled in by naval development. Its sister castle, Sandsfoot Castle is found across the harbour, west of Weymouth.