For thou hast been a shelter for me - A place of refuge; a place where I have found safety. He refers here to what had occurred in former times. God had protected him when in danger, and he pleads that fact as a reason why God should now interpose and deliver him. That reason seems to be founded on two considerations:
(a) God had thus shown that he had power to deliver him; and
(b) it might be expected that God who is unchangeable, and who had interposed, would manifest the same traits of character still, and would not leave him now.
Both of these are proper grounds for prayer.
And strong tower from the enemy - See the notes at Psalms 18:2.