Nothing wrong, we all make mistakes when learning
3D modelling is quite complex and it takes time and practice to learn it.
Press the N key. With the rotated part selected you should see in the numeric panel (that appeared when you pressed N) exactly which axis it is rotated and by how much . . . . . . . edit the numbers to tidy it up . . . .
. . . . but if you joined the bits together or applied the rotation, the figures will be zero.
It will also be zero if you rotated the parts in Edit mode rather than as objects . . . .
Best method if all the walls are the same is to delete the copies, then in Object mode
go to top down view (numpad 7), select the part to duplicate then do it all from the keyboard with shfit-D, return
(duplicate) then R 90 Z, return
for one face at right angles to the original: because you don't use the mouse there is no chance of odd rotations.
This will work best if you have the object center in the center of your planned tower: if it is not, the rotated part will not match up and you will need to fiddle with it.
When you have your four walls, join them to the original by selecting them, selecting the original last and pressing Control-J
The duplicates should have the rotation applied if you don't join them to the original so that they behave properly later with Control-A