The Strongroom Key is a key found in the level Barkhang Monastery in Tomb Raider II.
It can only be found once the Main Hall Key has been discovered, and the doors to the Main Hall have been opened. The Strongroom Key is found past a short avenue of slicing wall blade mechanisms, lying on a pedestal.
It unlocks the door to the Strongroom, a small room back by the main corridor, in one of the two library-like areas. In it lies another key, the Rooftops Key.