I recently played a game as the Celts, and I eventually won a domination victory in regent. Keys to success:
1) Build barracks early. They're expensive, so you want them to last by being veteran right off the bat. Barracks are really cheap anyways, it shouldn't be a problem.
2) Tech order goes as follows: Bronze Working, Iron Working. After that it really doesn't matter.
3) Get the iron hooked up quickly and start cranking them out. Being a man of culture it hurt to build Gallic Swordsmen instead of improvements, but it's necessary.
4) As for cities, have 5. That way you can 5 cities building GS's while other civs are building cities close to you. You're going to conquer them anyways, so let them build the cities for you.
5) When you have a decent stack of units, attack for all it's worth. If you're looking to capture cities, wait until the population exceeds one.
6) If you get a little behind, stockpile until you can do the free upgrade to Medieval Infantry. Get the extra attack and go for it! I found that Gallic Swordsmen armies are incredible as they can attack twice, and in all my wars using the GS UU I only lost about three units out of possibly 20, all of which had fought many battles.
Hope that helped!