Kampaeng Phet is a small city, but is on the main highway route between north and south. The most direct means of getting to Kampaeng Phet is by bus. There are regular buses from Bangkok, Chiang Mai and many other northern and central provinces. The trip from Bangkok takes about six hours and costs between 220 (5.71 USD) and 300 Baht (7.79 USD), depending on the service.
The closest airports would be in Phitsanulok or Sukhothai. Sukhothai's airport is served solely by Bangkok Airways, while Phitsanulok is served mostly by budget carriers. Either option would be about an hour and a half away from Kampaeng Phet by road. It's worth noting that there is very little bus service between Sukhothai and Kanpaeng Phet. In addition, Sukhothai's airport is more than a 30 minute drive from the city.