# my covenant faithfulness

The abstract noun "faithfulness" can be translated with anadverb. Possible meanings are 1) "the one who faithfully loves me" or 2) "the one who faithfully protects me."

# my fortress ... take refuge

The psalmist kows that Yahweh will protect him.

# my high tower

David speaks of Yahweh as if he were a fortress that protects him from attack. Yahweh is the one who protects David from harm. 

# my shield

David speaks of Yahweh as if he were a shield that protects a soldier. Yahweh is the one who protects David from harm.

# the one who subdues nations under me

"the one who enables me to defeat other nations"