Gladiators are trained by a barrack that is Level 7 or higher. They are the only units trained by a Barracks that can do a decent amount of damage to buildings, and are mainly used for that purpose as they don't do much damage to other units.
In battles, Gladiators are used for destroying buildings. They are the least effective Structure-destroying units, so they are not often used except in situations where Knights and Catapults aren't availible to the player.
|Training Ground||Fighter · Bowman|
|Barracks||Swordsman · Archer · Hoplite · Gladiator|
|Stable||Cavalry · Knight|
|Workshop||Helepolis · Catapult|
|Trading Post||Ox Wagon|
All unit stats by level can be viewed at the Official Help Database