2 Samuel 10:8

at the entering in of the gate This is generally supposed to be the gate of the Ammonite capital Rabbah. But the account in Chronicles states explicitly that the Syrian mercenaries mustered at Medeba, which was nearly 20 miles south-west of Rabbah, and clearly implies that it was the city which the Ammonites occupied. Further it is evident from 2 Samuel 10:9, which describes Joab as in danger of being crushed between the two forces, that the Syrians were at no great distance from the city. And how came the Syrian force to march past Rabbah to Medeba, if the Ammonites meant to make their stand at Rabbah? or how could Joab possibly have advanced to Rabbah, leaving this huge Syrian force in his rear?

Medeba, the modern Mâdeba, was four miles south-east of Heshbon, on a rounded but rocky hill. It existed before the Israelite conquest (Numbers 21:30), was assigned to Reuben (Joshua 13:9), and now seems to have been in the hands of the Ammonites. It is mentioned on the famous Moabite stone as having been recaptured by Mesha, and in the time of Isaiah was a Moabite sanctuary (Isaiah 15:2).

in the field "The plain of Medeba" (Joshua 13:9; Joshua 13:16) would be an advantageous place for the manœuvres of a large army, especially with chariots and cavalry.