An Interrogative in Psalm 127:1

 The Lord is unseen                    (Architects)?

The builders are seen.                (Laborers / Bricklayers / Builders)

The Lord is unseen.                    (Generals)?

The sentries are seen.                   (Foot soldiers)

There is unseen work that has to be done before any visible work is done.  Proper prior planning prevents piss poor performance.  II Corinthians 4:18 reminds that the things which are seen are temporal, but the things which are not seen are eternal.

If the unseen God has built a house, is that house able to eternally stand?  If the unseen God protects a city, is that city eternally fortified?

No. The city and house are seen and are therefore temporal.  They both will pass away.  While God gives the houses and cities for a moment, our focus ought to be on what lasts longer than a moment.  We are seen and God is unseen.  Like the city and house, we are temporal.

Unless the Lord builds a house, the work of the builders is useless.  Unless the Lord protects a city, guarding it with sentries will do no good.

We have to do what the Lord has already done, if our endeavors are going to succeed.