Hiya, good shot! For some advice, I would say think where your light is coming from before you begin your shading. At the moment, the light would have to be a big spotlight at the of the tower. If you wanted the light source from one side, then only one side should be shaded etc.
Second of all, take a look at the base of some towers and you'll be able to see the shape the bottom should be. At the moment the bottom is concave, you want it to be convex. In a more simple term, your's is an inwards curve, you want it to be a slightly outwards curve, almost flat to show it is sitting on the floor! Once you have done that, your brick lines should follow this curve.