Paying seven mana to flip the top three cards of your library and cast them for free is huge
Like most Magic: The Gathering cards, the value of Cast Down depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Magic Online Promos)
For example, Captain Storm, Cosmium Raider from the Lost Caverns of Ixalan release has an ability that says "Whenever an artifact enters the battlefield under your control, put a +1/+1 counter on target Pirate you control." If an artifact creature enters the battlefield under your control, that ability won't trigger
This retro frame version of the card is worth nearly $90 on the secondary market, though this writer prefers the artwork on the full-art Commander Masters version, as well as from an Artists Spotlight Secret Lair with art from Livia Prima: Also, this card is a major combo enabler thanks to granting other creatures the undying keyword: Long live Mikaeus
Tolaria West and Petrified Field are there just to make sure you can find and protect your Mistveil Plains
