If you make the Most High your dwelling... A reference to the beginning of the psalm. Look again at the first thought. I feel as though David refers us to the beginning, because he can imagine the doubts that his listeners have felt during the intervening verses.
A refuge is what we need in the middle of a storm or in the middle of a battle. A fortress is the place where we seek protection, and from which we fight the enemy. Whatever our needs in life, whatever we are running from, or fighting against, the Lord's shelter is the place to run to or to fight from. For He is the security we need in every circumstance.