Cannot be Normal Summoned/Set. Must be Special Summoned by a card effect. (Quick Effect): You can target 1 Machine monster you control; destroy that Machine monster and all monsters your opponent controls with ATK less than or equal to it. If a face-up EARTH Machine monster(s) you control, except "Machina Citadel", is destroyed by battle or card effect while this card is in your GY: You can Special Summon this card. You can only use each effect of "Machina Citadel" once per turn.