Neak Pean is not a temple. It sits in the the Baray of Preah Khan. It was constructed as a place of healing. It was believed that soaking in the waters at Neak Pean would balance the individual and therefore cure their illnesses. The center tower is round but the water is divided into 4 quadrants representing Earth, Wind, Fire and Water.
Access is limited at the time I was there. You can only walk about a quarter way around to the right or to the left. Many people ignore the signage and roped off area, walking where they like.