4 “You have led me down, because You have been made my hope: a tower of strength from the face of the enemy”. My heart is vexed, says that Unity from the ends of the earth, and I toil amid temptations and offenses: the heathen envy, because they have been conquered; the heretics lie in wait, hidden in the cloak of the Christian name: within in the Church itself the wheat suffers violence from the chaff: amid all these things when my heart is vexed, I will cry from the ends of the earth.
But there forsakes me not the Same that has exalted me upon the Rock, in order to lead me down even unto Himself, because even if I labour, while the devil through so many places and times and occasions lies in wait against me, He is to me a tower of strength, to whom when I shall have fled for refuge, not only I shall escape the weapons of the enemy, but even against him securely I shall myself hurl whatever darts I shall please. For Christ Himself is the tower, Himself for us has been made a tower from the face of the enemy, who is also the Rock whereon has been built the Church.
Are you taking heed that you be not smitten of the devil? Flee to the Tower; never to that tower will the devil's darts follow you: there you will stand protected and fixed. But in what manner shall you flee to the Tower? Let not a man, set perchance in temptation, in body seek that Tower, and when he shall not have found it, be wearied, or faint in temptation. Before you is the Tower: call to mind Christ, and go into the Tower....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)