Albert Barnes Commentary 1 Kings 15:22

"Then king Asa made a proclamation unto all Judah; none was exempted: and they carried away the stones of Ramah, and the timber thereof, wherewith Baasha had builded; and king Asa built therewith Geba of Benjamin, and Mizpah." — 1 Kings 15:22 (ASV)

Geba, situated opposite Michmash (1 Samuel 14:5), is almost certainly “Jeba,” which stands picturesquely on the top of its steep, terraced hill on the very edge of the “Wady Suweinit.” Its position was therefore extremely strong.

As it lay further north than Ramah, Asa may have considered that fortifying and garrisoning Geba would be a better protection for his northern frontier than fortifying Ramah.

Regarding Mizpah, we learn from Jeremiah 41:9 that Asa, in addition to fortifying the place, sank a deep well there to secure his garrison from a lack of water if the town were to be besieged.