This expression “tower of strength” is found in The Book of Common Prayer written in 1549, originally was used most often to refer to God and heaven:
“O Lorde … Bee vnto them a tower of strength.”
Shakespeare, being his own person, put a twist to the phrase in his play in Richard III written in 1594 where in Act 5, Scene 3:
KING RICHARD:
Up with my tent! Here will I lie tonight—
But where to-morrow? Well, all’s one for that.
Who hath descried the number of the traitors?
NORFOLK:
Six or seven thousand is their utmost power.
KING RICHARD:
Why, our battalia trebles that account!
Besides, the King’s name is a tower of strength,
Which they upon the adverse faction want.
But that’s not the first recorded use of the phrase “tower of strength.” In fact, the legendary ancient Greek epic poet, Homer wrote The Odyssey in 800 B.C. where the following is found:
“When I saw him I tried to pacify him and said, ‘Ajax, will you not forget and forgive even in death, but must the judgment about that hateful armor still rankle with you? It cost us Argives dear enough to lose such a tower of strength as you were to us. We mourned you as much as we mourned Achilles son of Peleus himself, nor can the blame be laid on anything but on the spite which Zeus bore against the Danaans, for it was this that made him counsel your destruction – come here, therefore, bring your proud spirit into subjection, and hear what I can tell you.’
So while I would love to give the prize to Mr. Shakespeare yet again, and while it might be refreshing to award the prize for this phrase to Homer, the phrase is a derivative of a phrase found in the Bible in Proverbs 18:10 where the following is written:
“The name of the Lord is a strong tower: the righteous runneth into it, and is safe.”
Historically Speaking 