Stormpeak is a pegasus cloudcity that was built into and around a mountaintop in Northern Equestria, now known as the Northern Wasteland. Stormpeak is first mentioned in Chapter 12 of Fallout: Equestria - Rising Dawn.
Stormpeak was one of the first cities built in the North. Constructed around and into a mountaintop, Stormpeak has significantly large portions of its city in which structures are built out of stone, unlike other cloudcities which are built almost entirely out of clouds.
During the Great War, Stormpeak was the base of operations for Equestrian air support. With a combination of Equestrian air superiority and Northern weather, the zebra advance was slowed significantly due to Stormpeak's contribution to the war effort. Because the zebras could not wrestle control over the air from the pegasi, Stormpeak's mountaintop and cloud defenses remained stalwart and impenetrable throughout the course of the war.
As the war came to a close, the zebras were able to break through Flankorage's defenses and advance upon Poneva, fighting at a stand still for many years. Stormpeak contributed heavily to the defense of Poneva city, and, when the line finally broke, the pegasi staged an evacuation of Poneva where many ponies were flown to Stormpeak to escape the chaos that ensued.
When the balefire bombs fell, Stormpeak was miraculously spared, as the balefire ICBM's that the zebras launched were aimed at Poneva and the Crystal Empire instead.
It can be assumed that Stormpeak still stands, today. Because of the persistence of the cloud cover over the Northern Wasteland, worsened and bolstered by the Frozen North's winds, it can also be assumed that the pegasi of the Enclave have inherited Stormpeak from their Equestrian ancestors.
Although the Enclave have yet to make a single outward appearance since the bombs fell in the North, the existence of dashites in the region - such as Night Sky, along with the fact that Stormpeak is the only cloudcity in the North, suggests that the pegasi of Stormpeak are still alive and well. Because many survivors of the sacking of Poneva were evacuated to Stormpeak, it is also quite possible that many earth ponies and unicorn descendants of said survivors live among the pegasi, under Enclave rule.