wall: This term has two metaphorical meanings:
A young girl's chest is flat like a wall (see Song of Solomon 8:10)
Until marriage, she should remain abstinent and strong like a wall
build a tower of silver upon her: Towers served as lookouts to protect the city; a silver one would be more noticeable and strong. If the woman is already a wall, having kept her virginity, they will honor and support her decision.
On the other hand, a swinging door suggests promiscuity. The brothers will make her control herself.