Yu-Shan is heaven, and the home of the gods. After the Usurpation, the Sidereals moved here as well, to allow the Dragon-Blooded to rule Creation. Yu-Shan contains all the office buildings of the celestial bureaucracy, the Tapestry of Fate, and the Jade Pleasure Dome.
Access to Yu-Shan from Creation is granted by several Gates scattered around creation (and one Gate that can be summoned anywhere). The Gates are guarded by the Celestial Lions, so using them without an invitation is not a good idea. Eclipse Caste Solars, as bureaucrats, can safely travel to Yu-Shan whenever necessary.