Unit leveling

I just started the game, is it not possible to gain unit xp and level up your troops?

That is how they had it before patch 13. They changed it to help with balance and new versus older players.

Damn it.. that was the best thing about this game. To be able to build up your army different from your enemies

You can still do that. You have 54 different units, nine heroes and they plan to add more heroes and factions. The only difference is they are pre-made instead of you customizing them. Also you don't have to spend hours leveling to find that your unit was better BEFORE you made changes... I think the addition of heroes was a big improvement. Now you build your team around your hero and what complements them.

units are nice right now