I do agree with ar7 that exploiting the AI reduces the fun of the game ( I've had experience with those exploitations. )
and as for the difficulty of taking Alesia you should try to build as many of the spear warbands ,they really kick ass , especialy versus warband infantry.
accompanied with a good general you shouldn't find it to difficult.
BTW it is possible to make money as Germania, just build as much economic buildings as possible. and try not to build more troops than you need. It's hard. the faster you take gallic settlements the more money you will generate