The various monuments around Kampaeng Phet were documented, stabilized and registered by the Thai Fine Arts Department between 1935 and 1937, and again in the 1960s. Further rehabilitation work was done in the 1980s, which is when the Historical Park was officially created. The park was listed as a World Heritage Site by UNESCO in 1991 as part of the inscription of Sukhothai, along with Si Satchanalai.
The historical park is divided into two zones. The first is the area of the old walled city, where the main sights are the ruins of the two main 'state' temples. The second area is the Aranyik, a forested hill just north of the walled city where you'll find the ruins of a dozen or more forest temples.
Park Admission: Admission to one of the zones is 100 Baht (2.60 USD) or you can buy a ticket good for both zones for 150 Baht (2.60 USD), but note the ticket is only good on the day purchased. You can easily see everything in one day, and see the sights outside the historical park areas on the following days if you have more time.
If you're staying in town, then I would suggest that you start with Aranyik, which is probably the furthest away from where you're staying, and then work your way back towards town. It's worth noting that, probably owing to the few visitors it gets, there isn't much food or refreshment available in the parks. You might want to plan on take a break between Aranyik and the walled city to find something to eat in town.