(Dragonskull Ampitheater isn't a spell.)
Dragonskull Ampitheater enters the battlefield tapped.
All creatures have Plainswalk.
: Add or to your mana pool.
Put Dragonskull Ampitheater on top of your library: Add to your mana pool.
: Add 
or 
to your mana pool.
to your mana pool. 
