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

[Caenwyr]

The black lines and dots represent walls indeed. Dykes are show as yellower/lighter colorations since they are higher than the surrounding land. You don't only see them at the coast, but also in the East and South, where they're crested with roads.

Basically there's a double moat, and the water level in both is close to the low tide level. The dyke between the two moats is high enough to keep the water out at high tide, but the outer dyke of the outer moat is structurally too low. If you open the outer locks (or lock, one would be enough) the surrounding land gets flooded when the tide comes rushing in. Opening the inner locks would have just minor effects, since the city is built on higher ground than high tide. The inner moat is there mainly as another defensive structure.

However, I'm thinking of changing that. Imagine a city that can be flooded entirely when the walls are breached. It would mean a massive obstruction for the enemy's march to the otherwise now poorly defended castle. People wouldn't have to suffer too much, the whole idea of inundation is to have a layer of water of a mere foot deep: too deep for an attacking army to advance rapidly on foot, too shallow for boats. None of the inhabitants should drown, and chances are their belongings would suffer less than when the enemy was actually able to sack the city. What do you think?