Building the Eternal Fortress

At the beginning of your upkeep, sacrifice {cardname} unless you tap three white creatures you control.
At the beginning of your precombat main phase, create a 1/1 white soldier creature token with vigilance, then put a Stone counter on {cardname}
When the third Stone counter is put on {Cardname}, sacrifice it and create a 10/10 Fortress creature token with Defender, Reach, Shroud, Indestructible, and "This creature may block any number of creatures"