The warlord castles are already upgraded and can't be upgraded again and therefore not repaired. I also get the impression that the flooding mechanic stops wokring if you load a saved game.

No, I have not tested it in multiplayer. I will have a look at it. Thanks for the hint.

Did you test this map in Multiplayer if yes doesnt the Flooding create a desync and become invisible ? i made my own map with scripts and it seems scripts just cause desync in multiplayer so maybe you know something i dont know