After a break will be starting a Roman Campaign, which difficulty lvl should I play?

[Kralizec]

Well, maybe some background information from my campaign would explain it.

The Sweboz first attacked me from the alps. I was allied with the Averni. They were getting rapidly exterminated by the Aedui and Sweboz. I intervened: destroyed the Aedui, drove the Sweboz out of Gaul and gave the Averni most of the Gallic provinces. The motivation was that I really didn't want to conquer Gaul in an early stage for myself; I wanted to be able to face the Gauls in late game.
I also took those two cities in the Alps (the two directly north of Italy) from the Sweboz so that I had a buffer against them, consisting of Type IV client states. Because I had beefed up the Averni considerably by now, and becaus I had launched a couple of punitive raids against the Sweboz, they never became much of a threat again. They still attacked my two Alp cities regularly but rarely with full stacks. Usually I could decimate a besieging army in one turn with mostly slingers. It's an annoyance to be sure, but nothing major.

[Titus Marcellus Scato]

I also took those two cities in the Alps (the two directly north of Italy) from the Sweboz so that I had a buffer against them, consisting of Type IV client states. Because I had beefed up the Averni considerably by now, and becaus I had launched a couple of punitive raids against the Sweboz, they never became much of a threat again. They still attacked my two Alp cities regularly but rarely with full stacks. Usually I could decimate a besieging army in one turn with mostly slingers. It's an annoyance to be sure, but nothing major.

Did you not consider giving the two cities in the Alps to the Arveni or Aedui instead? The Sweboz AI will not attack you if it doesn't share a border with you, hence creating buffer states controlled by other factions can be more useful than creating Type IV governments - the AI doesn't care what the government level is, it regards those towns as yours and the AI tends to regard the human player as the primary target, especially on higher difficulty levels.

[Kralizec]

I'm aware of that, but I didn't expect that the Averni would be able to keep those towns.

For various I often go out of my way to prop up my allies. In my experience if you save a faction from extinction by returning key provinces to them, they're still not be competitive with their enemies and will usually lose their newly gained cities in a couple of years.

Giving the Alp cities to the Averni and making peace with the Sweboz would merely have made the Averni their main target, and probably would have lost some of the Gallic cities again that I worked so hard to acquire for them. Because I stayed at war with the Sweboz however I divert some of the forces that would otherwise have been sent against my ally.