2 Chronicles 14:7
Therefore he said unto Judah, Let us build these cities, and make about them walls, and towers, gates, and bars, while the land is yet before us; because we have sought the LORD our God, we have sought him, and he hath given us rest on every side. So they built and prospered.
While Judah enjoyed peace and the land still lay open before them, Asa led the people to use the time to build and to strengthen their cities. He called them to seek the Lord while they undertook this work of strengthening. The strengthening meant surrounding the cities with walls, towers, gates, and bars. Because they had sought the Lord their God, He had given them rest on every side, and so they built and prospered.
These defenses were not made to take the place of trust in God. When invasion came, Asa did not rely on his fortifications or on his army, but relied on the Lord. The invasion he met in that dependence on God was by an Ethiopian army.