This time I've found another image of the floor plan of the castle, and I think I understand it. It is still almost rectangular but more detailed this time (I've made the underground passageway and the aqueduct) and put other things to "decorate" the castle.
The rest of the map I filled it with a river and some green areas.
This time it's only a landscape, so if you want an invasion play it in castle-builder mode and create more bows, when you're ready press F1