Adam Clarke on Psalms 125:2

As the mountains are round about Jerusalem - Jerusalem, according to Sandys, was situated on a rocky mountain every way to be ascended, except a little on the north, with steep ascents and deep valleys, naturally fortified. It is surrounded with other mountains, at no great distance, as if placed in the midst of an amphitheatre; for on the east is Mount Olivet, separated from the city by the valley of Jehoshaphat, which also encompasses a part of the north; on the south, the mountain of Offiner interposed with the valley of Gehinnom; and on the west it was formerly fenced with the valley of Gihon, and the mountains adjoining. The situation was such as to be easily rendered impregnable.

The Lord is round about his people - He is above, beneath, around them; and while they keep within it, their fortress is impregnable, and they can suffer no evil.