Assuming time machines are possible to build, then it is a good assumption that at some future point in time, whether it's hundreds or even thousands of years from now, a time machine will be built. Then, somebody will get into that time machine, travel back into the past, and mess it up. Thus, someone from the future has probably already messed up the past.
In fact, this new castle in Arkansas may be a symptom of the damage already done to the space-time continuum. Frickin time vandals!
The bigger question is, did anyone do something like this in the past and seriously fuck with the archaeological time line?