I kinda deviate from the lesson and follow another road while keeping the mirror modifier on. I removed the faces on the bottom of the tower and did on each side of the tower a loop edge cut with 2 subs and extrude them, then fill the faces under after I did little dissolve edge work before. Hope you like the final result 
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Looks fine.
Lots of ways in Blender, just bear in mind the course is showing you a more basic method that works for most of the items without the mirror modifier.
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