Albert Barnes Commentary 2 Samuel 10:8

"And the children of Ammon came out, and put the battle in array at the entrance of the gate: and the Syrians of Zobah and of Rehob, and the men of Tob and Maacah, were by themselves in the field." — 2 Samuel 10:8 (ASV)

Came out - From their city, Rabbah (Deuteronomy 3:11; Deuteronomy 3:15), which was 15 or 20 miles from Medeba, where the Syrian army was encamped (1 Chronicles 19:7).

Medeba (modern Madeba) was taken from Sihon (Numbers 21:30) and was assigned to Reuben (Joshua 13:9, 16). In the reign of Ahaz, it seems to have returned to Moab (Isaiah 15:2), and in the time of the Maccabees to the Amorites . In church history, it was a bishop’s see.

In the field - That is, in the plain below the round, rocky hill on which the city stood.