(Ancient Den isn't a spell.)
: Add to your mana pool.
Darksteel Citadel is indestructible. ("Destroy" effects and lethal damage don't destroy it.)
(Great Furnace isn't a spell.)
(Seat of the Synod isn't a spell.)
(Tree of Tales isn't a spell.)
(Vault of Whispers isn't a spell.)
: Add 
to your mana pool.