The Guildhall of the Alchemists is the headquarters and training center of the Alchemists' Guild in the city of King's Landing. It is located underground, beneath Rhaenys' Hill. Tyrion Lannister visits it after discovering the Guild was preparing wildfire at the behest of Queen Cersei.
In the books
In the A Song of Ice and Fire novels, the Guildhall of the Alchemists is the center of the Alchemists' Guild. It is located at the foot of Visenya's Hill, along the Street of Sisters in King's Landing. The Guildhall is made of black marble, a warren of twists and turns.
Wildfire, the material that makes the Alchemists' Guild infamous, is prepared in stone cells beneath the guildhall. Above each cell is a room filled with sand. The floor below has protective enchantments placed upon it. Any fire within the cell will cause both floors to fall and the sand to drop, smothering the fire instantly.