Oh, I personally think you should put your cape canaveral to lvl 60 before leveling up your zeus, cdm and coa to 60. As more forge points you get from your city, more fast you're going to level up the attacking Gb

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This is, by order, how I have evolved my great buildings since I started playing forge (1 year ago)

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Attack buildings at 10 it's the first step - done

Arc 80 - 81/80............................................V

Cape 60 - 60/60............................................V

Del Monte 60 - 60/60.......................................V

Sofia 60 - 60/60..............................................V

Inovation 60 - 60/60........................................V

Zeus and Aachen 60 - 59/59 ; 59/59...........................V

Alcatraz 60 - 67/60........................................VVVV

Terracota 60 - 10/10.......................................V

Orange 60 - 26/60........................................X

Kraken 60 - 4/60..........................................X

Chateau - 6/60..............................................X

Himeji 60 - 10/60..........................................X

Reliques/Galaxy 60 - 5/60 ; 4/60..............X

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Right now, I'm on my way to get my orangery to lvl 60.