Building Architecture (Spoliers)

Forgive me for asking this but I'm pretty visually-spatially hopeless and I'm also unfamiliar with old apartment buildings like this one.

Was it that there was a bunch of dead space in between her apartment and his, or was this actually another apartment that had been left to rot? How did Max access ipthis space - from his apartment, or another? I remember his apartment was an end unit and hers was down the hall with several in between so I don't see how everything connected.

Max's hiding spots and the setting for the end confrontation certainly had the feel of the bowels of a building - narrow, winding, etc., but just how everything fit together was unclear to me. Was it all on the same level or did anything go to the basement? For the different vantage points (I.e., bathroom, kitchen) it would seem that there would have been more than one area like this. I don't exactly know the layout if her apartment but I don't think everything was linear - the kitchen and the bathroom were likely in different planes (geometrically speaking).

I know this doesn't really matter to the plot but I like to understand things.