Hi Spart,

I've only been playing for about 1 year, but using Britannia I got very strong, very quick by using certain tactics against the Gauls (obviously different tactics for different enemies )

Gaining experience on the battlefield seems to be from different issues:
your whole army can get more experience from a massive victory over a larger army, especially your General, which is obvious:

But each unit can gain from its own personal battles usually from defeating a stronger unit - using 2 units to hit from the front and back ( alternating rather than at once so only one unit is actually fighting) works wonders, or being lucky and attacking a unit just before it routs; using horse to shock attack the back of a fighting unit also works wonders; routing units or decimating but with skill rather than strength is the aim - no point wiping out peasants with your Heavy Infantry ( except for the fun of it).

If the bar at the beginning of a battle showing the chance of victory is against you, and your expected to lose and through your genius leadership kick some arse; thats what its all about!

Good luck!!