Within the greater complex of Yogyakarta's Water Castle is housed a fascinating site that requires no entrance fee and takes you way back to the 17th century. The Underground Mosque looks more like a cistern and isn't at all easy to find. Get one of the friendly locals to show you the way through the maze of alleyways just outside the Water Castle itself. The alleys themselves are a sight to see.
The mosque has two levels – traditionally, one level was for men and one for women – and is striking because of its simple yet sleek design and the way light enters the structure.
If you like photography you will certainly like this place as well as the many other downtown Yogyakarta places of interest.