127:1 builds the house. The primary reference is to the actual building of a residence, but as a “Song of Ascents” written by Solomon, it is natural to think specifically of the temple. Furthermore, in the light of the second half of the psalm, the house refers to the family as well as to a physical structure.
watches over the city. A city is not secured by its defenses, but by the protection of the Lord. The psalm would remind pilgrims that Jerusalem was not a place of magical protection and security, but that all depended on the Lord.