Suppose I want to have my Magnificent Mansion be accessible via a trap door in a ceiling (which one would climb up into) or a cellar entrance (which one would climb down into), rather than the more traditional vertical doorway. Is there anything preventing me from doing so?
The relevant text from the spell is:
You choose where [the mansion's] one entrance is located. The entrance ... is 5 feet wide and 10 feet tall.
The use of the word "tall" vaguely implies that the entrance must be vertical like a regular old boring door, but that hardly seems conclusive.
I'm mostly asking this for flavor reasons, but I suppose the answer also has some mechanical implications, since one could, for example, create an invisible trap by putting the entrance on the floor of a hallway and then opening it while the quarry walks over it, causing them to fall in.