First off, I'm gonna suggest you pop the columns out and use octogonal 2x2 bricks for the tower columns. Enhancing these details is an easy way to convey the architecture of the building:
For the slit, I'll propose a SNOT assembly with vertical plates. There are several ways to achieve this, but since I can see some 1x1 plates with thick ring, here's a possible assembly (including a jumper plate):
The assembly rests one brick below the window baseline, and the jumper plate makes it possible to align it properly. You'll need some internal scaffolding, though.
Here's a view from the back to illustrate this alignment:
If you prefer the slit to be dark instead of see-thru, you can use a 1x3 black plate.
And, of course, remember that this is not the only way of building a SNOT assembly for the slit - I'm sure you can make alternative arrangements that should be easy to hide inside the tower.