SolarMission Technologies (OTC: EVOMY.PK,  ASX: EVM.AX) and its Australian subsidiary, EnviroMission Limited produced this 5 minute video about its early pilot plant in Spain. It is an older video (2000) but gives a basic understanding of the solar tower concept. The designs have changed and technologies added, but it gives an introduction to the solar tower concept.
http://thefraserdomain.typepad.com/energy/2007/03/solarmission_so.html