Ivorytusk Fortress

• WWizards of the Coast2014-09-20
  Official Ruling
  Each creature you control with a +1/+1 counter on it untaps at the same time as the active player’s permanents. You can’t choose to not untap them at that time.
• WWizards of the Coast2014-09-20
  Official Ruling
  Effects that state a creature you control doesn’t untap during your untap step won’t apply during another player’s untap step.
• WWizards of the Coast2014-09-20
  Official Ruling
  Controlling more than one Ivorytusk Fortress doesn’t allow you to untap any creature more than once during a single untap step.

Rulings via Scryfall