I didn't have a lot of time to work on this last night because I was demoing WarMachine for someone, but I think I'm done with the first step.

I'm debating on whether or not to use some wood filler over the top of the flat surfaces. It would provide a more durable surface, as well as smooth out some of the areas I've circled. It may depend on how expensive the wood filler is, since I don't already have any.

Either way, I think the top and the steps will need a stone look since it's too round to look like a natural feature. Maybe something that looks paved.