God's being the psalmist's strength represents God protecting him. AT: "you are my strength" or "you are my protector" (See: [[rc://en/ta/man/translate/figs-metonymy]])
A high tower is a place that people could go to for shelter from their enemies. The psalmist speaks of God protecting him as if God were a strong, safe shelter. AT: "you protect me like a high tower" (See: [[rc://en/ta/man/translate/figs-metaphor]])