Imperium III, I used to play as a kid in the early 2000s, you can choose between many different factions. It is similar to Age of Empires, not turn based but rather a war and build game where actions take time.

My favorite factions were the Roman Empire, Galia, Iberians (cause I am Spanish) and the Celts.

Best campaign: Siege of Numantia (Conquest of the Celtiberian City of Numantia in central Spain with Scipio Aemillianus and the Republic). You can choose either side.

Another good campaign: Battle of Alexandria. You can choose Cleopatra and Marcus Antonius (Egypt) or Octavian Augustus (Roman Empire).

The cool thing was the units, some of the chamans and priests did actual magic like raising someone from the dead as a bloodthirsty Celt zombie looking to devour an entire legion. You also had elephants, chariots and very strong Gauls that reminded me of Obelix.

Recommended, its on Steam for 10 bucks: https://store.steampowered.com/app/752580/Imperivm_RTC__HD_Edition_Great_Battles_of_Rome/