hm, I always call him "A(y)-Dee-Wan" ;-)

well for the Snow-Troops, there are cut scenes, where they enter through side-tunnels or the ones the Wampas broke into the base (hence making the scene with C3PO afterwards, where he rips the warning-sign), if I am correct.

But the main conception is, they enter mostly through the seen doors (not saying other openings could exist).

As fot that, we never see Vader land with his "dropship", but it isn't needed because it would slow down the pace of the battle/evacuation :-)