The Colossus is a new unit to Starcraft 2. It basically replaces the Reaver from the original Starcraft. It also has the ability to scale cliffs with easy, being able to walk up and down the cliff. This enables it to make a quick escape or a decent surprise. It is also subject to ground and air fire since it is classified as a Massive unit. It even has an additional range upgrade called Extended Thermal Lance, which provides siege range. The Colossus is available from the Robotics Facility, but requires a Robotics Bay to be built.
