Siena Stone BIG, BOLD AND BEAUTIFUL The latest retaining wall system to be introduced by Unilock combines the stability of large modular units with a rockface appearance. Steps, curves, corners and angles can be easily constructed with Siena Stone and erected with light construction equipment. Under ideal conditions Siena Stone retaining walls can be built to approximately 6 feet (1.8 meters) in height. The use of geogrid soil reinforcing techniques will allow Siena Stone retaining walls to be built to even greater heights. In addition, octagonal units are available for an even bolder, sculptured look allowing you to make curves.