Irriac, before and after the inundations used to lift the siege

[atpollard]

Castles and cities are built for different reasons and with different priorities.

Stone walls require strong foundations and the strategic ‘high ground’ is paramount for defense of a castle.
So the castle will occupy the military strongest defensive point, displacing whatever it needs to.

The city serves an economic purpose with ‘defense’ as only one of several important factors that need to be balanced. Resources need to flow to shops/factories. Finished Goods need to flow to other markets.

The ability to flood the streets and countryside seems like an interesting strategic advantage for the defenders who can optimize their militia and tactics to maximize that advantage … soldiers trained to fight from flat-bottom punts, buildings with second floor living quarters that can serve as sniper posts.

As a side note: with 1 foot of water, dirt streets may turn to mud and make travel difficult, but paved streets will render a foot of water little more than a minor nuisance … waist to chest deep water would be more effective at forcing the invaders into boats (creating small, vulnerable clumps of men subject to hit and run ambushes). Perhaps deep roads and shallow alleys, forcing the invader to choose between advancing through narrow spaces (which favor the defenders) or splitting the force into small boat-sized units (easily ambushed by crossbow snipers).

It is an unusual defense, but not unimaginable.

PS. A surprise attack by an army large enough to storm a castle seems unlikely. You should have plenty of time to flood the city before an enemy can take control of the locks ... and it only takes one small breech in the dike.