If so, my siege workshop and barracks have kind of.. combined? My great barracks has moved to the spot where my siege workshop was and a forge is in it's place as far as I can remember. I may have got this slightly wrong as I can't remember the exact layout I had, but the problem is.. I no longer have a siege workshop. The barracks has kind of taken over it and both building graphics are on top of each other, although when I click on it, it only takes me to the barracks.
If you haven't added a new slot, then one of my buildings has randomly deleted.
This is in my village 'Andy'.

