Khu Bua (คูบัว) is an ancient town dating back to the Dvaravati times of the 6th to 10th century. It was abandoned in the 10th century, and 4 centuries later the town Ratchaburi was founded nearby.
I used this town as an example in my main blog for the fact that the historical towns (Mueang) were relocated rather easily, with only the few stone-built structures left to decay more slowly. While actually Khu Bua and Ratchaburi were not a simple relocation, the main temple Wat Khlong illustrates beautifully what does remain of such a town after 1000 years.
The main ruins are along Thai highway 3338, the large parking lot cannot be missed. The easiest way to go is to follow highway 3339, which starts near the railway station in the center of Ratchaburi town. Right at the parking lot is also a small museum, which sadly was closed for renovations when I visited. From what I found on the web the artifacts found at Khu Bua are on display in that museum and the National Museum in Ratchaburi. I don't know if any other ruins are as clearly visible as this Wat Khlong, but actually there are a total of 44 sites within the area.