Albert Barnes Commentary Nehemiah 4:13

"Therefore set I in the lowest parts of the space behind the wall, in the open places, I set [there] the people after their families with their swords, their spears, and their bows." — Nehemiah 4:13 (ASV)

The lower places — the places where those within the walls had the least advantage of elevation, the naturally weak places, where an enemy was likely to make an attack.