The Apex of Babel

What does it mean to 'build The Tower of Babel' and 'play ... atop this' ?

You may play The Apex of Babel onto the same square as The Base of Babel! (If you have multiple copes of The Base of Babel in the realm, choose one.) If you do, they merge into a single site called The Tower of Babel. When you build it by playing the Apex, resolve the effect on The Apex of Babel. The Tower of Babel is considered a single site that provides (2) mana and (A)(E), and is both Unique and Exceptional. (The turn you play it, you gain (1) more since the Base was already in .) If it's destroyed, both and go to your . If it's copied, e.g. , the will provide (2) , etc., but won't get the build bonus again. For all purposes, The Tower of Babel is considered a single .