defense
衞 centers on protection and guarding: the act of defending against harm.
衞 is the traditional form of 衛. It combines 行 (to go, movement) with an inner component that likely relates to guarding or encircling. The exact historical development of the inner part is uncertain.
The outer 行 suggests movement or a path, while the inner part looks like a fortified area. Picture soldiers marching around a perimeter to defend it.
For エイ, imagine a defender shouting 'Hey!' to warn off intruders: 'Hey!' -> エイ, as they guard the perimeter.