Renwick's Ruin is the remnants of an abandoned hospital used by Valentine Morgenstern and other members of the Circle as a hideout while in New York. It contained a Portal, before the Portal was destroyed in 2007.
Renwick Smallpox Hospital was a famous lunatic asylum, debtor's prison, and hospital built on Roosevelt Island, intended to quarantine the poorest victims of the smallpox epidemic that broke out over Manhattan in the 1800s. It was designed by Jacob Renwick on Roosevelt Island, then known as Blackwell's Island, owned by the Blackwells, an old Shadowhunter family. Eventually, the hospital was abandoned to disrepair and public access to the ruin is forbidden.
Surrounded by a high chain-link fence and floodlights, strong glamour makes the hospital look like ruins to mundane humans. Valentine used it as a hideout in the City of Bones, where he kept Jocelyn and had Forsaken warriors as his security.
Although the "bones" of what the hospital once was is still visible, it was heavily renovated by the Circle to their taste, looking like, according to Clary, an ornate Gothic Revival structure; more like a castle than a hospital. Patterns taking forms such as those of roses from Idris, runes and rayed suns are visible around the structure. The Circle's Latin motto: In Hoc Signo Vinces ("By this sign, we will conquer") is carved on the floor as a rune.