The crusader sea-castle at Sidon. It was built in the early 13th century upon a small, low island and connected to the mainland by a fortified bridge (the present one is more recent). It is built in part from Roman building debris, including shaft columns. The collapse of Roman Sidon occurred in the mid-6th century, the result of the 551AD earthquake that also levelled Beirut. So the crusaders recycled earthquake debris. The site itself probably result from the earthquake as it caused systematic coseismic uplift all along the Lebanese coastline (manifest as raised notches). The low island that forms the base to the sea-castle possibly popped out of the sea at this time.