We drove up to a lookout (free that day due to the UNESCO opening ceremonies) and took lots of pictures. The square one was built first they are easier to build but take more material and provide less light thru the roof. As the builders got more skilled more round and oval ones were made.
This link below has the best information. The Wikipedia can give you a much better history of each cluster than I can. I was too busy taking pictures to pay attention to all the history stuff I quickly read on the signs and heard.
http://en.wikipedia.org/wiki/Fujian_Tulou
We got to see the first 5 listed on the Wiki page. We saw the Chuxi tulou cluster, Tianluokeng tulou cluster, Gaobei tulou cluster, YuChang lou. And many other ones I don’t know the name of.
