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 mana since the Base was already in play.) If it's destroyed, both The Apex of Babel and The Base of Babel go to your
cemetery. If it's copied, e.g.
Mirror Realm, the
copy will provide (2) mana, etc., but you won't get the build bonus again. For all purposes, The Tower of Babel is considered a single site.