Complex sentence: Afterwards, it was used as vegetable garden, use that ceased in 1996, when the cistern was nationalized and transformed in a concert arena for pop music with a capacity of 12,000.
Split into simpler sentences: Afterwards, it was used as vegetable garden until 1996. In that year the cistern was nationalised and trasformed in a corcert arena for pop music with a capacity of 12,000.