The Po Nagar Cham Towers is one of the many relics of the Champa Kingdom, which was the most significant power in South Vietnam from 200 - 1720 AD. Despite being a major trading civilization, it never weilded a great deal of power, and so never really competed with the Khmers in the south or Viets in the North.
The Cham civilization had spread from Ancient India, and it is interesting to see some similarities between the design of Cham monuments and some others within India.
In Nha Trang the Cham Ponagar temple complex (locally refered to as Thap Ba) is situated on a hill just outside the city, and is reached by crossing the Cai River from the main tourist stretch.
At the complex there are four towers remaining from an original eight, each of which shows differences in style, indicated they were built over an extended period between the seventh and 12th centuries AD. The largest, which stands at 23m high, was built in 817 AD and contains a lingham and a statue dedicated to Ponagar, or Lady Thien Y-ana, the beautiful wife of Prince Bac Hai.
It is said that Lady Thien Y-ana taught new agricultural and weaving techniques to people of the area, and in gratitude they built the tower to honour her.
Other towers in the complex are dedicated to gods - the central tower is in honour of Cri Cambhu, and is thought to bless devotees with fertility, so is visited by childless couples. In the North West lies a tower dedicated to Sandhaka, a wood cutter and foster father to Ponagar, and the tower in the south was dedicated to Ganeca, the daughter of Ponagar.
Entrance to the towers costs 12,000 VND and opening hours are from 6 AM - 6 PM. Late afternoon is a good time to visit.
Source: http://www.vietnamvisasupport.com/f@rum/content/289-po-nagar-thap-ba-cham-towers.html