Without seeing your game its impossible to be sure, but I suspect that the square you wanted to put it was unavailable, and there is something in the game that automatically puts it in the nearest one, alternatively it maybe a random selection of 1 of the 4 squares that there are from a 2 by 2.
This being so, each time you tried you would have only a 25% chance of getting the actual square you wanted.
It could also be something to do with the programming and the graphics that determine placement.
Perhaps people with more experience and memory of what happened to them will be able to help you.
I think you must have resources and scrolls to burn if you destroyed the stronghold just because it wasnt exactly in the place you requested.
I have been having a closer look.
The stronghold covers the 4 squares in the block, and I am assuming none of the one you built occupied 256 589, so it would seem that it moved your request one square to the left, so I would take a guess and suggest in order to get it where you want ,that you need to choose one square to the right, i.e.256 588, but I cannot guarantee that this square is available, nor if it will work .