Cavalry are mounted troops, so a Stable is required before any cavalry units are available.
Cavalry units are generally faster than Infantry, though a combined force moves at the speed of the slowest unit. They also tend to be more expensive in research, training and upkeep, though they also tend to be more powerful.
Gaul cavalry are fast, with the Theutates Thunder moving at 38 tiles per hour in speed3x and 19 tiles in others. The Haeduan are moderately strong attackers and faster than average cavalry, but their outstanding quality is as defenders against cavalry, strength 165, 57% stronger than the strongest Roman and more than twice as strong as any other Roman or Teuton unit. They also have the Druidrider who is a very good defense against infantry and also very fast providing excellent reinforcement where it is needed.
Roman reconnaissance troop Equites Legati can manage a speed of 32 fields per hour, but cannot carry any resources. Roman light cavalry Equites Imperatoris can manage a speed of 28 fields/hour with a very solid attacking power of 120 and can carry 100 resources. The heavy cavalry Equites Caesaris has a speed of 20 fields/hour, a devastating attacking power of 180, and can carry 70 resources.