Messing around with the CP and noticed Festung Helgoland runs out of supplies quickly. I tried shipping two supply wagons there to build a depot but it won't allow me to build one. Had 3 transport ships in one stack and the 2 supply wagons and a marine division in another, neither stack could build a depot...
So the depot needs to be added to the map on setup I think, since it's very annoying to have to deal with trying to keep it in supply, not fun at all and just busy work.
Jim