Greencastle. A seaside town and fishing port on the east coast of the Inishowen peninsula of County Donegal. The name is taken from the castle there which was built by the then Earl of Ulster, Richard de Burgh, in 1305. The castle was originally called North Burg Castle, later becoming New Castle and finally Green Castle whose name it is thought is in reference to the green-tinged stones with which it was built. The ruins of the castle are still pretty impressive and stand at quite a height when viewed from the lower side. Beside the castle, atop high walls, is a Martello Tower which dates from the start of the 19th century and was built by the British to guard against invasion by Napoleon. A twin Martello tower is across the water at Magilligan (in County Derry).