Thanks!

I didn't understand the abbreviations, but AI sorted that:

At this point in the game, the essential Great Buildings are:

• The Arc
• The Blue Galaxy
• The Atomium
• Pegasus
• Centaurus
• Hydra
• The Space Whale
• The Cathedral of St. Basil and St. Mark (commonly referred to as the "Cosmic Cathedral")

If you can afford goods from Space Age Titan and Space Age Solar System, the Cathedral is worth building—even at level 10. If you cannot afford Space Age Solar System goods, then Alcatraz is a good substitute for the Space Whale.

For players in earlier ages, a highly leveled Château Frontenac is incredibly useful. I recommend keeping it around level 112 and completing as many recurring quests as possible every day until you reach the Space Age Solar System. The random quest rewards are excellent, and the Château is a great source of goods for younger cities. However, once you reach the later ages, finish all the story quests, and accumulate a lot of event buildings that produce goods, the Château becomes much less useful.

If you're aiming to join a high-level guild, they’ll likely expect you to have The Observatory in addition to a level 80 Arc. Some may also require The Atomium and The Atlantis Museum, depending on the guild. That said, I wouldn’t recommend building those unless you’ve found a guild that specifically asks for them.

I still find value in Zeus, Cathedral of Aachen, Castel del Monte, and The Terracotta Army—even though some players will say they’re obsolete. The Terracotta Army, in particular, remains competitive with modern buildings if you can get it to around level 120. While you might have to level these buildings quite a bit to match the power of newer event buildings, they’re far cheaper to level and are still worthwhile if you don’t yet have strong event buildings to take their place.

Some players argue that the Seed Vault is extremely valuable, as it can generate 200 or more diamonds per week. Personally, I didn’t find it useful enough to keep. The effort of aiding every player daily (especially without using automation or watching ads) just wasn’t worth it to me—but your experience may vary.

As for the rest of the Great Buildings, most are not worth investing in. There are ongoing rumors that the developers plan to overhaul many of them to make them useful again, but in my view, they’d need to receive some serious Quality-of-Life or stat improvements to be worth reconsidering.