Thrace when defending really benefits in a large battle from using a roman manipular formation.

P= Phalanx pikemen, F=Falxmen, B=Bastarnae, A=Archers, S=Skirmishers

___________PPPPP_____PPPPP_____PPPPP_____PPPPP__________
______FFFFF_____FFFFF_____FFFFF_____FFFFF_____FFFFF_____
SSSSS_____BBBBB_____BBBBB_____BBBBB_____BBBBB______SSSSS
______________AAAAA__AAAAA__AAAAA__AAAAA_____________

Against phalanx based enemies, they will tend to task two phalanges onto each of your pikemen block. However this means that they come in on angles, exposing their flank to your falxmen (who should have no difficulty hacking them to bits). If the falxmen break, then the bastarnae need to go in.

Against non phalanx enemies, it's a little more brutal. It basically is hold until they hit your phalanx line, then surge the falx men forwards.