Verticality
Also said as High ground, Vertical level design
Using height as part of the layout, so that where you stand up and down matters as much as where you stand across. Whoever is above sees more and has less exposed to them.
A flat map offers two dimensions of decisions and a tall one offers three. High ground gives you something to fight for, and a dangerous route up can pay for itself with what you can see from there.
Say it
“Give the arena a balcony 4 m up, reached by one exposed staircase. It should see two thirds of the floor, and anyone up there can be seen from it too.”
Where you’ve seen it
Batman: Arkham Asylum's predator rooms put gargoyles above armed guards. From a perch you can see them all and pick who to take next, and on the floor you would last seconds.
Not to be confused with multiple floors
Floors stacked on top of each other with no way to see or shoot between them are several flat levels. Verticality means the heights can affect each other.