Several of the islands strung across the South entrance of Castle Harbour, Bermuda were fortified in the early days of the territory, hence the harbour's name. When official settlement of the archipelago by England began in 1612 (unofficial settlement having begun with the 1609 wrecking of the Sea Venture) the first permanent town, St. George's (then called New London) was placed on the North side of St. George's Harbour. St. George's Harbour could be accessed directly by channels from the East.
Those channels, however, were shallow, suitable, originally, only for small ships. As a consequence, and despite any major settlement on its shores, Castle Harbour was an important anchorage in the early years of the colony, with its main entrance, Castle Roads being an important route in from the open Atlantic for shipping. It was also a weakpoint, as it was remote from the defences of St. George's Harbour, and difficult to reach. It was quickly fortified and garrisoned by a standing militia.
Initial fortification by the Virginia Company's (Bermuda was originally settled as an extension of Virginia) first deliberate settlers in 1612, was around the inlets to St. George's Harbour, but by the end of that year work had commenced on Castle Harbour's defences, starting at Castle Island (previously called Gurnett's Head, and King's Island). As a temporary measure, two guns had been salvaged from the 1609 wreck of the Sea Venture, one of which was installed on Castle Island ) in 1612. Proper fortifications were soon raised under the instructions of Governor Richard Moore. King's Castle is, today, the oldest surviving English fortification in the New World.
Their fortifications are the oldest surviving English New World fortifications (due primarily to their being constructed of stone, whereas contemporary English fortifications on the North American continent were built from timber and earth). They were also the first English coastal fortifications in the New World. Those then existing were depicted and described in the 1624 book, The Generall Historie of Virginia, New-England, and the Summer Isles, by Captain John Smith. As a result of their historical significance, they have been made a UNESCO World Heritage Site, together with St. George's Town and other nearby fortifications, like Fort St. Catherine (UNESCO World Heritage Site: Historic Town of St George and Related Fortifications, Bermuda).