I’m currently building the large city of a region that will be the focus for my next campaign.  The city is located on the Eastern Coast of the Continent, with a river running west to east and the ocean through the city, effectively bisecting it.  Perhaps another quarter of the city’s land or whatever is located on a pretty close island where one of the city’s two temples is located, along with the main castle-palace.  I imagine that a fair bit of the eastern shore is dedicated wharfs.  The City is surrounded by walls perhaps 40 feet tall, with the river entrance defended by chains above and below the water.  On the northern half, there is a smaller river (but still wide) that split from the larger and serves as a moat between the city walls and the country side.  The southern half of the city is (so far in my imagination) simply land that extends to farms and cottages past the walls.

I’m curious if people feel this layout is defensible and would make sense for a location to build a city in the first place.  Any extra steps the military would have taken to defend it, either with structures and fortifications or large scale “landscaping.”  Would they have diverted part of the river to further wrap around the southern walls for example, or would this mostly be unnecessary?  Any suggestions are appreciated.  I know I want roughly 70% of the city estates to be on the mainland, with an effective “island” in the sea (it’s not really an island, it’s been connected by bridges).  Thoughts about how I can complicate things in it’s design are also appreciated.  Note that this is obviously still early stages as I mold things in my mind’s eye.

It’s tough to say. How common are tritons, sea elves and other intelligent creatures with a swim speed? Or just access to spells or items that allow breathing water? How common are casters who can teleport/dimension door to the gates to open them? Folks with stone shape who can casually walk up and make a 5’ hole in the walls?

That's a really cool city idea - it reminds me of Ankh-Morpork! I think it would be cool to make the defences mirror the idea that the city is bifurcated. And yeah, of course there could be a city here. Definitely add a whole ocean aspect here - maybe for the island part, the defences are all underwater and there are secret underwater passages for the semi-aquatic citizens? And perhaps the walls are built from basalt found from the coast and glued together with a kelp-and-saltwater-infused cement? Also you could have the southern half of the city have the basic defences found in all cities - big wall with guards. But all of this also depends on the size of your city. Okay, the city is large, but even just a large city might not have amazing defence. If it were a metropolis, then the walls would be heavily guarded and so would the underwater passages. This also means that it would have a larger and more diverse population, so aquatic races need more ways of travelling than just walking.