I haven't had much debacles playing the Brits. The chariots took some time to get used to, especially manouvring them before they engage 'cause they mess up your own "formations" as well, be it not lethal. (and, silly enough, I thought Chariots needed stables, but it was Blacksmiths! so for about a decade, my only chariots were family members and the light ones from the begining, such ignorance!)

I mostly left the Germans alone, except for the occassional punitive expedition when they were naughty and tried to take Alesia. I Plundered trier and had it rebel.

I did not wait for the Gauls to attack me and made my move first, taking Condate Rendonum and Alesia, and the rest of Gaul soon followed. Use your slingers to draw their cavalry at your line and overwhelm them with warbands, then regroup and deal with their infantry. These should be attacked by warbands in the front and chariots in the rear. the will rout within seconds if you can make such a combined attack.

I haven't done a lot of direct charges with generals bodyguard, but a rear-attack with any type of chariot is usually enough to break the enemy, 'cause the enemy is hit suffering 2 things that damage morale: 1. the chariots themselves, and 2, the fact that it's in their rear (or flank) while engaged to the front by warbands. Most of the time, this rear attack is so effective that it's effect (enemy rout) is immediate, meaning your general can butcher at will without getting damage.

after Gaul I finally got to conquering Hibernia (Ireland), which was quite easy... and then the Romans (Julii) came knocking at my gates.

Using "devide and conquer" tactics, succesfull ambushes and the meagre garrissons of Italia cities to my advantage, I conquered Italy in a few decades. Italy is an important province to conquer for all factions, but especially for the barbarians 'cause Rome is almost full-tech and you'll have the all important chosen swordsmen!

I try to NEVER fight siege assaults if I can manage, especially as barbarian and even more so against the Romans. When I'm the besieger I enjoy it most to have an enemy relief force come and be destroyed, but my main object in such a battle is always the garisson of the besieged city. If you manage to wipe out the garisson troops joining the battle and beat the relief force, the city is yours!

If I really need to assault I use up to 5 rams for wooden walls and pallisades and try to deploy in an area where there are the fewest towers.
Against stone walls, I use only siegetowers and sap points (though, towers mostly). Boiling oil is terrible, I try to stay clear. I usually have 2 towers with elite troops attack the wallsections beside the gate, so I can conquer the gate and have the rest of my army walk in securely, and another tower as a diversion or sneak attack. If the gate is conquered, i do not rush the city square before the towers I will pass heading there are conquered, this prevents casualties on my side, and if the enemy comes to reclaim their gate, will damage the enemy!