in‧vade, v. /ɪnˈveɪd/
From an older form of “vision” with the “in-” prefix. (Note the retained archaic construction “invasion.”) When an army has invaded a stronghold, it can see the insides of it. From without, all that can be seen is the outer walls.
An “outvasion” has been recorded only once in history. Very quick-thinking defenders leapt from the outer walls of Schloss Schwangau and broke the opening mechanism for the portcullis, trapping the invaders inside.