Sorry for the seemingly trivial question, but none of the YouTube vids I've watched on the subject have revealed anything.
As if it weren't bad enough that the Select button is required to exit the tank (against common NES convention - although I could also fault the convention, for limiting the scope of that button to menu selection only... in most games).
But let's get on with it. There is a door, and I am in front of it. Yet I have no idea how to enter it. What button combo am I supposed to press?
On the other hand, if this is a matter of me lacking a certain key required to pass, then that's fine. Just give me some user-feedback to indicate that. I'm pretty sure that's not the case here, though.
In most games of this era a simple UP on the D-pad would suffice. But here we have something of a usability mystery. Please give me a clue... or even a totally in-depth explanation. ;-)
Blaster Master is a little different; you need to press DOWN on the d-pad to enter doors, whether on foot or in the tank.