It was enclosed by a 50 foot high wall roughly 3.5 by 5.5 miles.
After I visited the site in 1999, they found stairs that suggested the inhabitants were 7 - 9 feet tall. They also found a rock with a couple of metal rings embedded in it. What I found interesting about the metal alloy is that it contained tin. The closest tin mines to Dallas are located in Alaska and the UK.
Currently there's no site to visit. The dig was done down one side of the wall. After a rain, the soil expanded and collapsed the wall. They covered up the site and were not able to afford another dig.
Here are some links I've collected about Rockwall:
This shows the dig close to the time I was there
Has write up from the group's now defunct website
Map of the wall
Tidbit - there are other walls east of Rockwall
Some early history
I've only briefly looked at these YouTube videos, but they appear to relate what I saw and information gathered from those actively part of the dig (and then some):
Presentation Given In Houston (long - 2:27 in length)