How do you get gladiator in Tower Defense?

Can you elaborate on the process of acquiring the "gladiator" unit in the Tower Defense game? Is it available from the start, or do you need to unlock it through certain achievements or in-game currency? Are there any specific requirements or strategies that one must follow to successfully acquire the gladiator? Additionally, is there a time limit or level restriction associated with unlocking this unit? Understanding the mechanics behind acquiring the gladiator would be greatly appreciated.