The Koules or Castello a Mare is a fortress located at the entrance of the old port of Heraklion, Crete, Greece. It was built by the Republic of Venice in the early 16th century, and is still in good condition today.

The fortress is made up of two parts: a high rectangular section, and a slightly lower semi-elliptical section. Its walls are up to 8.7m thick at some places, and it has three entrances. The fort has two stories, with a total of 26 rooms, which were originally used as barracks, a prison, storage rooms, a water reservoir, a church, a mill and a bakery.

A lighthouse tower is located on the northern part of the fort.