Help with a room

Heya so i'm not a great builder i often do the strict minimum and i had a lot of inspiration when i saw the pools paradise challenge it's a first to make such a big building and i wanted to know why the room isn't registering itself.

Is it because i terraformed a lot and build in close to the castle or that i didn't filled some holes to the top, to make the floor because it would take too long ? Or maybe is it because i build it close to the castle ?