The Fortezza is a massive Venetian fortress on the headland above Rethimno's harbour, built in the second half of the 16th century to defend the city from Ottoman raids. It is one of the largest Venetian fortifications in existence — a vast walled enclosure covering the entire headland — and the views from its ramparts across the old harbour, the city, and the Aegean are among the best in Crete.
The interior of the fortress is a large open space, partly ruined, with the domed shell of the Ibrahim Han Mosque at its centre (converted from a Venetian church after the Ottoman conquest) and various other structures in various states of preservation. Walking the full circuit of the walls takes 20–30 minutes. The fortress is open daily; early morning or late afternoon visits avoid the tour groups. The walk up from the harbour is steep but short.