My visit to this site was rather marginal. During a stay in central Crete in the summer of 2018, I explored Heraklion for about half a day. Included in the itinerary for that excursion was a stop at the fortress of Rocca al Mare, or Koules, which guards the city harbor.

The cost to enter is fairly negligible and the required time allocation does not exceed an hour, so even though it is not rising to the level of exceptional, on balance it is a reasonable point of interest to explore. The interior spaces hold a small historical and archaeological exposition (in addition to offering a cool respite from the summer heat). The ramparts offer excellent perspectives onto the town and out to the sea. It is from the ramparts that you can best observe the remains of Heraklion's town walls along its quays. The glass-covered dozen or so skylights in the roof add a bit of unwelcome modern touch to the impression made by the fort.

I have mixed feelings about fortifications on the WH list when they do not present as a fully integrated ensemble with the city that they are meant to protect. Based on this minimal evidence, I do not feel that Heraklion's fortifications are worth the WH status, but I obviously have to reserve my opinion of the full site until I visit other proposed locations.