"The Solid" is a symbol of the primordial, unchanging Foundation of the Divine Plans, Intents and Destinies, approved by the goodness of His sacred Will.
"The Solid" is an image of the steadfastness, indestructibility of the eternal Wisdom of God's Laws of Life, defining Good and Evil, Truth and Falsehood.
It is the fullness of the all-encompassing diversity of their manifestations in the Perfection, Beauty and Harmony of all that exists. 
Negative Meaning.
"The stronghold," the "strongholds" of sinfully living man is a symbol of false religious, political, philosophical and other foundations of false doctrines, ideologies, dogmas, on which he relies, trusts, is guided in his life.