Changi was originally built by the British as a prison. After the Japanese invasion in 1942 the occupiers took over the prison and combined it with the nearby British Army barracks to create a War Camp.
About 50.000 POW's were detained here but there were also civilians, specially Dutch who were captured in Indonesia, by then a Dutch colony.
Nowadays there isn't much to be seen on the spot. The original prison was demolished and although there is a Changi Prison, it's located in a different area and it's totally new.
There is however a small museum, which includes a replica of the camp's chapel and a nice exhibit dedicated to the suffering of the POW's there.
Check here the excellent Wikipedia article for further info:
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visit link)