The Adamant Citadel is the home of the Iron Sisters. The Citadel is well-protected, hidden, isolated, and is open only to the Sisters and other female Shadowhunters who are free of demonic impurities.
The Citadel is reached primarily through Portal. Portaling will lead outside the gates, which is surrounded by mountains all around.
Visiting Shadowhunters cannot enter the Citadel itself without passing the purity trial because of the fragility of the adamas they keep. The heavenly energy in the water outside the Citadel used in the trial identifies those with demonic impurities and destroys any contaminant upon contact, with some limitation.