The Scofield Bible Commentary, by Cyrus Ingerson Scofield, [1917], at sacred-texts.com
castle of Zion
Hebrew, castle.
(1) Zion, the ancient Jebusite stronghold, is the southwest eminence in Jerusalem, called in Scripture the city of David, and associated with the Davidic royalty both historically and prophetically (Ch1 11:7); (Psa 2:6); (Isa 2:3). The word is often used of the whole city of Jerusalem considered as the city of God (Psa 48:2); (Psa 48:3) especially in passages referring to the future kingdom-age; (Isa 1:27); (Isa 2:3); (Isa 4:1-6); (Joe 3:16); (Zac 1:16); (Zac 1:17); (Zac 8:3-8); (Rom 11:26). In (Heb 12:22) the word is used symbolically of heaven.
(2) In (Deu 4:48) the name is given to a projection or peak of Mount Hermon.