The first documents testifying to the existence of a castle in Castro date back to the 13th century: a document of 1282 described it as having a strategic importance to protect the kingdom. It is quite sure that the castle was built on the ruins of a former Byzantine building, which protected the fortified Roman village. In 1480 it was destroyed and then rebuilt and strengthened  in 1572 by the Spanish viceroy.
On the north-western part, stand the thick walls of the Bastion of the buttress, with a single central room and fortified battlements. The Torre Cavaliera, the highest and most imposing one, is a three-story tower with three rooms, covered with barrel vault. The Torre Circolare, dating back to the Aragonese period, has three rooms with small windows and merlons with machicolation.
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The Broch of Clickimin is a large and well preserved, though somewhat restored broch near Lerwick. Originally built on an island in Clickimin Loch (now increased in size by silting and drainage), it was approached by a stone causeway. The water-level in the loch was reduced in 1874, leaving the broch high and dry. The broch is situated within a walled enclosure and, unusually for brochs, features a large 'blockhouse' between the opening in the enclosure and the broch itself. Another unusual feature is a stone slab featuring sculptured footprints, located in the causeway which approached the site. Situated across the loch is the Clickimin Leisure Centre.