Like a city whose walls are broken down is a man who lacks self control (v28).
A city whose walls were broken down was vulnerable to attack. With no functioning defensive wall, it would be much easier to overrun and plunder.
In the same way, self control is our defensive wall from the onslaught of Sin. If our 'walls' are weak or broken, our resistance will quickly fail and we'll be ravaged.
So how do we strengthen our 'city walls'?
One of the fruits of the Spirit is 'self control'. It's not that we are to produce self control, rather we are to pursue walking with the Holy Spirit. He will produce this fruit in us and we then faithfully exercise this gift.
None of us should want to be overrun and ravaged by Sin. But we can't do this on our own. We are not supposed to. We need to humble ourselves and be reliant upon the Holy Spirit as He equips us to live for the Glory of God.