A gateway is a monumental structure which frames an opening or doorway. It consists of two main supports linked at the top by a span, which may or may not join the supports seamlessly. A gateway must either be a freestanding separate structure or a distinct section of a city wall (which cannot be construed as a guard tower). It may abut another structure as long as that structure is clearly distinct from itself.

The two supports and upper span define a passage rather than an enclosed room. The span can join the supports continuously or remain visibly separate from them.

The gateway can stand alone or form a distinct wall section. Abutting a clearly separate structure does not remove its identity as the framing element around the opening.