Darksteel Citadel

• WOTC Official Reminder Textofficial rules
  Indestructible — Effects that say "destroy" don't destroy this permanent. A creature with indestructible can't be destroyed by damage.

• WWizards of the Coast2014-07-18
  Official Ruling
  Although Darksteel Citadel is an artifact, it can't be cast as a spell. Playing it follows the normal rules for playing a land.

Rulings via Scryfall