The Gard is built and carved into a mountain in Idris, with its doors protected heavily by wards impervious to opening runes. It has cells for holding prisoners, both Shadowhunters and Downworlders, charged of crimes, or awaiting trial or execution. It has a morgue. It also has a courtyard where a culling of its death row inmates takes place every month.
A while after Valentine Morgenstern was held captive in the New York Institute, his transfer to the Gard was requested by Alec Lightwood and later approved, but his transfer to the city was intercepted.
The warlock Iris Rouse, several incarcerated Circle members, and later Clary Fairchild were imprisoned in the Gard and put on the death row. Many were executed in their monthly culling. Valentine was brought back to life with Clary's Necromancy rune in the Gard's morgue, and he later almost escaped after massacring the guards. Clary was able to open the doors to the Gard with her unlocking rune, and though she had no intention of escaping, she was broken out by Lilith's Owl.