The most famous tower in the Old Testament is Babel, meaning "God's gate." It was man's attempt to reach God on his own ability. This failed. But there is another tower that can help us reach God, and that is the Tower of David—Mary.
The tower of David is a reference to Mary's physical beauty, her strength, security, steadfastness, and inaccessible womanhood. Vigilance and ascent are other attributes of a tower. We need to be vigilant in the spiritual life; on the lookout for pitfalls and sins that will lead us away from God and make us fall back into ourselves and, ultimately, into Hell. We are, instead, to ascend upwards to God. We cannot do this on our own, but only through the assistance of the Blessed Mother. The Tower of David rises high into the Jerusalem sky.
'Ivory Tower' (turris eburnea) is a related Marian title. Ivory invokes delicate beauty, a celestial structure, dazzling whiteness, resplendence in the sunlight, lovely in shape as well as in substance, elegance and stainlessness. All of these are qualities of the Blessed Virgin Mary. A tower of ivory is for the king. So too is Mary's soul for Christ the King, a place of purity and beauty that brings him delight. Mary's heart, in which Jesus could rest, is pure, calm, peaceful, and lovely.
Finally, castle: another related image for Mary. The castle is the most secure and protected place in the city, to be defended at all costs and to the last. All cities used to have, at its center, a fortified castle or citadel. Mary, the Castle, cannot be overcome by the Enemy. We protect the castle, but, in reality, the castle—the heart of the city—stands over and watches out for the city, for us.