Vespasian is a Caesar Kingdom prestige hero and requires 150,000 prestige to unlock.
Unit Type - Gladiators: High Dodge Rate. Gambit - Iron Curtain: Deal full gambit damage (Entrench is ~half) to a single target and enter a defensive stance for 1 turn, reducing all Normal and Gambit damage taken to 1 (2 if critical).
Iron Curtain does full gambit damage unlike Entrench which deals about half of your Gambit Attack value.
Before you take the above as "Vespasian is a god", he's not. His stats are good for his prestige value, but his unit type could be better and he's still vulnerable to spell attacks through entrench, and will eat up your priests blessings some of the time which are often better spent on your heavy hitter gambit hero.
Overall he's the best entrencher around but he suffers the same weaknesses. Entrenchers are niche heroes (meaning they're great in some situations, terrible in others), no matter how good they are, they're a gamble which you'll lose more often than not.
Vespasian gains 34 units per level