City of Shadows

• WWizards of the Coast2004-10-04
  Official Ruling
  The storage counters aren't removed when you activate the mana ability.
• WWizards of the Coast2009-10-01
  Official Ruling
  You can activate City of Shadows's second ability while it has no storage counters on it, though it won't add any mana in this case. You did not tap it for mana, though, in case any abilities (such as the ability of a Fertile Ground enchanting it) care about that.

Rulings via Scryfall