This Thai National park is the gateway to Khao Phra Whihan in Cambodia (see also Prasat Preah Vihear) from Thailand. The entrance to the Temple Ruins is generally open to everyone and no visa is required but on occasion is closed due to Thai-Cambodian political tensions.
The highlight of the park (aside from going to the temple) is a nearby cliff in Thailand just east of the promontory on which Khao Phra Wihan resides which allows a good overall view of the temple complex about 1/2 mile away in Cambodia.
Clarification
Khao Phra Wihan and Prasat Preah Vihear are exactly the same place, the differences in the names arise from three sources, mainly the difficulty in translating Thai or Cambodian into English, secondly the differences in the name in either Thai or Khmer, and thirdly the difference between the physical name and the proper religious name. Khao Phra Wihan is the place where the Temple is located, and is Thai-based. Prasat Preah Vihear is the Temple itself and is Khmer-based.