Metalworker by tapping it and revealing at least three artifact cards from your hand, adding at least six ({C} magic symbol) .
Activate
Urza by paying ({6} magic symbol) , creating a token copy of
Darksteel Citadel that is a 1/1 Soldier.
When the
Darksteel Citadel token enters the battlefield,
Retreat to Coralhelm triggers, untapping
Metalworker.
Repeat.
If you have four or more artifact cards in hand, this combo nets infinite colorless mana, and you may target any artifact you control to get infinite copies of them.