It actually makes sense that there are two hatches in that design. The top hatch would be used for top-down gantry-way access on Star Destroyers which we have seen in the films, and the rear hatch would be used for planetfall access. There's probably a collapsible ladder on the low-side of the hull. I couldn't imagine trying to get in and out of that thing while landed using its top hatch. Which actually makes me wonder how they get in and out of AT-AT's and AT-ST's without additional external superstructure assistance.
Always loved this site, although I do agree with Sojourner that the top header should be shrunk. Takes up way too much static vertical space.