The SNOT parts on the corners are attached with 1x1 with stud on one side. Each wall is connected by hinge bricks, 1 stud behind the surface of the wall that is showing. And finally, the walls are braced by the pieces on the corners as well as the structure that attaches the arrow slits.
This tower and its scenery look like they could be real, except for the studs all over the ground. None of the details are insanely showy, which is a good thing in this case. I like the corner technique especially, and, like Deus, I wouldn't mind knowing how you connected the six walls.