+
|\
| \
| \
| \
+-o+o-+ +--+ +-o+
| | o\
+-o+--+--+o+ \
o \
+ \
\
+-o+o-+ +
| | | o
+--+ + +--+
o | o 4
+ + + /
| +
o /
+-o+--+--+--+
4 = Mega-Maze Basement (Part 4)
Here is the universal key.