Workshops build the units of battle. Each workshop can only build a few units, and they must be aligned with the workshop, so choose carefully. Unfortunately, if a workshop is destroyed you will loose the ability to build anything that workshop was currently producing, until you build another workshop to replace it.
When a workshop creates a unit, a Storm Power stream streaks from the workshop to the build site*. When the power stream combines with the proper energy the unit will be immediately created.
Rain workshops produce rain units. They have twice the hit points of Sun Workshops and are slightly more expensive.