Casse is definitely an interesting campaign and one I enjoyed a lot.

I can confirm what LastDays says, they have brilliant heroic infantry units, but most are recruited only in Britain (except the Rycalawre).Kluddargos is an impressive unit the type of Romphaiaphoroi (or Guild warriors :-P) which can cut through "Roman heavies", easily.

The special about Britons is their generals who ride their chariots.These units have their pros and cons like they spread fear in enemy units around them (with a large AoE), have awesome charge bonus, BUT are not good to be left to fight in melee especially alone.In fact after a lot of battles, having learned how to play those units I could turn the tide of battles just by having them lifting morale (my army) and spreading fear (enemies).

Against the other gaulo-celtic tribes I felt Casse were superior in terms of units and tactics.Both Aedui and Averni were easy to conquer, only their generals exceled IMO.

Late game you get into a conflict with both Lusos and most probably Romani.That's where the game gets serious ;-)