This isn't located in Gensokyo, but I'm writing about it due to its close relation to Eientei.

The capital of the moon is said to be a great, ancient metropolis on the far side of the moon.

According to the stories about the Genso-Lunar War (*1), this is the city that the youkai of Gensokyo invaded.

It's understood from what records are left from that time that their civilization is extremely advanced.

It is essentially impossible to get to the lunar capital now, but we can gather something about the current state of the lunar capital from the exhibitions held at Eientei.

The lunar citizens possess weapons with a firepower that far surpasses anything here on Earth.

Youkai would surely be unable to stand a chance against weapons such as a rifle that can shoot a spread of many bullets in an instant, or small arms whose rounds cause great explosions, or guns that shoot bullets that the user can control after firing (*2).

Indeed, in light of such things as medicines that can cure all illnesses and lengthen lifespans, or sauces that can make dumplings ten times as flavorful, it's clear that not just their weaponry, but the whole of their civilization is more advanced than that of Gensokyo.

It's unknown how long the citizens of the moon have been living there, but it's said that they live forever.

The people of the moon consider the Earth to be a residence for the corrupted, and it's said that they use the Earth as a prison for those who commit serious crimes.

1: See the article on Yukari Yakumo.

2: These various articles are said to be from long ago. Modern lunar civilization has surely advanced even further.