Venetian & Ottoman Heraklion, what to see
While in Crete, you should discover the largest town on Crete and it's hidden gems. Heraklion in the past was known as Rabd Al-Handak (Arab period), Khándax (2nd Byzantine period), Candia (Venetian period) and Kandiye (Ottoman period). The locals often called it simply Megalo Kastro (the Big Castle).

This 3 thousand year old town has one of the biggest fortresses in the Mediterranean. The Venetians fortified Candia with walls that have 7 bastions and a large moat:

Here's a virtual tour of Venetian Candia:
Ottoman monuments:

Ottoman fountains:

Aghios Minas catherdral:

St. Titus church/mosque:

The Morosini fountain:

The Armenian church:

The tomb of Nikos Kazantzakis:

And of course don't forget to try some local coffee at a traditional café.