FLOATING CITY FORTRESS
The Aztec capitol was famously situated in the middle of a lake. Surrounded by chinampas - artificial plots of farmland - a combination of clever diplomatic maneuvers and the natural defenses offered by the water worked to make their capitol unassailable from all known threats, at least until European contact.
Anyways, Tenochtitlan didn't have walls, but that's only because sieges can't happen without pack animals for complicated logistics reasons. Your fantasy world probably does have pack animals, and therefore a proper city requires walls. So here's a set of Aztec-inspired city gates maps, with walls, on the water.
MANY MUCH VARIANTS
This map comes with the following major variants:
All variants are as follows:
License: Roll20 End User License Agreement
(Personal Use Only, Do Not Distribute)