Charles Ellicott Commentary Psalms 87:1

"His foundation is in the holy mountains." — Psalms 87:1 (ASV)

His foundation. —This abrupt beginning with a clause without a verb has led to the conjecture that a line has dropped away. But this is unnecessary if we neglect the accents, and take gates of Zion in apposition with His foundation:

His foundation on the holy hill
Jehovah loves, (even) Zion’s gates,
More than all Jacob’s dwellings.
Here His foundation is equivalent to that which He has founded, and the gates are used by metonymy for the city itself. (Compare to Jeremiah 14:2.)

With regard to the plural, mountains, it is probably only poetical, though geographically it is correct to speak of Jerusalem as situated on hills. Dean Stanley speaks of “the multiplicity of the eminences” which the city “shares, though in a smaller compass, with Rome and Constantinople” (Sinai and Palestine, p. 177).