The folder path is Rome Total War/bi/world/maps/campaign/barbarian_invasion

Inside is the file descr_strat.txt. Open it, and cut and paste Celts from non-playable to playable.

Thanks!

As a side note this only works for factions that are present at the start of the game. For example it won't work for the Romano British. If you want to play as an emergent faction you have to use Epistolary Richard's All Factions mod.

And you can download that here.

One thing that disappointed me was that I could not become a horde as the Celts. (I found out the hard way by leaving my towns and letting them revolt, and lost the campaign lol)

That's not how you horde. If you have one city then you select the "horde" button in order to horde your faction.

You can only horde with the Franks, Alemanni (?), Goths, Vandals, Huns, Sarmatians, and Roxolani. And I know I've forgotten one - probably a faction not there at the start of the game.
And even if you could horde with the Celts (whatever they are supposed to be in this game, at least) the North Sea would stop you from getting very far

Alemanni (?)

Cannot horde, no.

And I know I've forgotten one - probably a faction not there at the start of the game.

Slavs. You also forgot the Lombardi and Burgundians.

And even if you could horde with the Celts ... the North Sea would stop you from getting very far

So you build boats first, of course...

Lombardi and Burgundians

As I found out horribly on a Saxon campaign...

And even if you could horde with the Celts

Hmm... am I right in thinking that you could mod it so that a faction can horde?

I was just thinking that. It would be relatively simple. Basically, after editing a few files, you would just have to clone a few units for hording. Horde Spearmen, Horde Gallowglasses, Horde Chariots... you get the idea.

[This message has been edited by Andalus (edited 12-05-2008 @ 06:01 PM).]

I thought so. I might do that tomorrow when I'm bored all day, due to having to stay at home all day :/.

Hording is easy to turn on.

In Descr_SM_Factions.txt you must add the following code between custom_battle_availability" and "can_sap"

horde_min_units60
horde_max_units100
horde_max_units_reduction_every_horde 20
horde_unit_per_settlement_population300
horde_min_named_characters4
horde_max_percent_army_stack 80
horde_disband_percent_on_settlement_capture33
horde_disband_percent_on_settlement_capture50
horde_disband_percent_on_settlement_capture100
horde_unit[horde unit]

You can have any number of horde units listed. The more often a unit is listed, the more of them will spawn when the faction hordes. Use the Type line in Export_Descr_Unit.txt to denote the units.

The horde symbol that appears on the unit card must be manually added.

Horde units need the "can_horde" attribute in Export_Descr_Unit.txt. They will still horde without it, but will not disband when the horde settles. You also have to manually set their upkeep to 0.