I know it's not the most interesting of structures - I probably should have added more projections and cut-aways but the look of the building wasn't as important to me as to trying to convey the size of the thing.
If I were to make other changes, I'd make the forest more jungle-y behind and to the right of the structure. I like how the vegetation is growing on the thing, but it could have been more so, but as it is, it makes you wonder if the structure (is it a tower or gate or lived-in building?) is going to ruin, or of we have made friends with the plants and can now live in harmony with our chlorophyll filled brethren. I also like the really small, smudges that could be interpreted as birds. I can almost hear them.
One thing I worked hard on this one is the high contrast part right at the base of the structure. I wanted the viewer's eye drawn here, which I hoped would make one feel more pulled into the drawing, into the tower's opening.
And then there are the happy accidents. Who doesn't love 'em?! Heck, they are what got me drawing more and more. Check out the trees at the bottom right. See how the shading is comprised of horizontal, not diagonal lines? Believe it or not, I've never done horizontal shade lines. I think it really works here. Those lines both suggest leaves but also push the vegetation further back into the distance.