There was a mosque under the land of Sumur Gumuling , Yogyakarta. It actually a fortress that being build 1765 , but been used as mosque for the Muslim take their prayer until 1812 after another mosque been build at Keraton or the castle. Believe it or not , the wall thickness is about 1.25 m.
It was a circle shape without roof at the top of the mosque. At the middle of it , there was a well with 5 stair toward at the middle of it symbolizes the number of pillars of Islam. The mosque is he legacy of Sri Sultan Hamengkubuwono I.
