Although the Observatory is not a required building for the game, it is a desirable one to own. Each level of order produces a higher level of building resources.
In order to build the Observatory you must complete a set of quests:
"Astrologers" starts the Observatory Quest Set
Unicorns are found in the Shop > Decoration tab.
You need all three unicorns for the observatory.
"Sky Keeper" Reward used in building the Observatory
1/5
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Hover on the different orders below to see the initial Build Requirements + Upgrade costs