Your starting point is the Koules Fortress, the Venetian sea fortress at the entrance of the old harbour of Heraklion — about a 10-minute walk from the city centre bus station. This is self-guided: no guide meets you. Just start at the fortress and follow the route on your own phone.
The route finishes at the Natural History Museum of Crete, on the seafront west of the old town. From here it is about a 15-minute walk back to the harbour and the city centre.