Great job! Thanks for sharing this.

When I installed the castle, there was no entrance - the staircase was turned from main door to some point in the air between tow towers. Finally I've put stairs to entrance, though, only two cases wide, cos "crossing with the base is impossible". Didn't understand why is so.

These things probably happen because some of the houses I uploaded were build in an earlier version of the game.
In later versions (after some updates) some of the building restrictions changed. So some structures that where possible to build a certain way in the past are now impossible to build that way.

For example building a pool next to a raised base (like the moat of this castle) is also no longer possible unless you raise the pool to the same level.
So in my game i just live with the fact that the courtyard wall is now invisible from the outside (luckily it still shows on the inside).
I do not have a solution for it unfortunately (other than removing the pool).

I think something like that is going on with the staircase as well. Placing an object in the empty spot (with help of bb.moveobjects on) to make it look better might be the easiest option.
I also looked into this problem in my own game. The diagonal wall on the front left of the tower seems to cause this problem. The stairs can be placed if the wall next to the stairs is straight instead of diagonal.
- One option is to pull the bridge platform back (one block) add the strairs and make path leading up to the bridge one block longer.
- Another option is to remove the 1/2 block triangle (including the diagonal side next to the mailbox), then replace the deleted part with a 1 block square plattform and build walls on the outside (or build a square 1 block
room).