Jamieson, Fausset and Brown on Psalms 18:2-3

2-3. The various terms used describe God as an object of the most implicit and reliable trust.

rock--literally, "a cleft rock," for concealment.

strength--a firm, immovable rock.

horn of my salvation--The horn, as the means of attack or defense of some of the strongest animals, is a frequent emblem of power or strength efficiently exercised (compare De 33:17; Lu 1:69).

tower--literally, "high place," beyond reach of danger.