Many Elven Fortresses weren't built by masons and carpenters, but created from the living forest itself.

Elven Fortress

{1}{G}: Target blocking creature gets +0/+1 until end of turn.

Fallen Empires

o1oo
G Target blocking creature gets +0/+1 until end of turn.

"The size of the obvious Fortress walls often misled foes. Actually, the Elves enchanted the forest itself to provide the first line of defense with tangling vines and stinging thorns."
—*Sarpadian Empires, vol. III*

"Thallids are not ordinary enemies of flesh and bone and reason. They attack with no thought for our strength or of their own losses. I fear our Fortresses shall be overwhelmed."
—Kyliki of Havenwood

"Burn it down, you say? Now *there's* a stupid idea. What do you suppose will become of those overhanging branches, hmm? And then what do you suppose will become of us?"
—Ivra Jursdotter