Simtokha Dzong, known as ''Sangak Zabdhon Phodrang'' (Palace of the Profound Meaning of Secret Mantras), is said to be the oldest surviving fortress cum monastery established in 1629 by Zhabdrung Ngawang Namgyal, who unified Bhutan. It was attacked several times in the 17th century but survived and was refurbished repeatedly. It is a small dzong (only  square with gate on the southern direction), located about  to the south of Thimphu. It now houses one of the premier Dzongkha language learning institutes, namely "The Institute for Language and Culture Studies". Both monks and common people study here. Students who graduate from this school primarily become Dzongkha teachers.