The protected valley

 The area I was in was a huge valley, completely surrounded by hills except for where the two rivers passed through them. The area in the picture above has been rice paddies for around 1,500 years and was farm land for another 1,500 years before that.

This is one of the rivers, a stream really, that cuts through the surrounding hills. Now of course, there are many houses in the area but people have been building in the valley longer than they have been farming. The hills in both pictures have the remains of forts which protected the only two access routes, the rivers. The hills, although not too high, have very steep sides and effectively block movement through them.