Ok, basically this is what happened:
At the start of our multi-layered session I picked Provence.
After establishing how the situation was in Europe and after having my wife "removed" and replaced by a younger wench I proceeded to take Arborea (north Sardinia). Having a stable economy (around 14G per month) and a army of 3K I invaded muslim Southern Spain, taking four muslim provinces, including Cordoba.
Meanwhile Germany has ceased to exist as a cohesive state and the germanic duchies were fighting amongst themselves, that along with a small egyptian (?) expeditionary force creating havoc around central Europe.
As Germany crumbled and the Von Franken heir turned to Burgundy, I took Piedmont and put all the french provinces in the hands of faithful vassals and removed myself to Iberia (the provinces there were much richer and gave a higher income while still within my demesne limit.)
At the height of my "power" this was the general view of the Duchy of Provence (numbers indicate combined military manpower of that region's provinces):
While half of my provinces in France where fighting the Count of Piedmont, Burgundy (the former Kingdom of Germany and still nominally my liege) declared war on the Emirate of Seville that in turn declared war on me.
Having man in southern Spain I decided to ship those in Northern Sardinia and the remained of the French forces (roughly ) to Southern Spain to assist in the upcoming invasion of Seville's territory (or so I hoped).
The muslim forces spread out and attacked my northern territories first. I couldn't move my main army to defend the north because that would leave the central and southern provinces just open to the main muslim army so I waited for my forces crossing the Mediterranean to deal with the northern invasion.
The Muslims waited a couple more days and gave me time to assemble a army of around 6500 men while my sardinian army marched north to meet the very small northern invaders (around 200 versus my 800).
And this is when everything went potty shaped...
The Muslim main army, which consisted of 2000 men, attacked the province where I had my main army.
I had 6500 men, my marshall had level 17 which combined with my military skills gave me 27 in terms of bonus, and my province had a minor hill fort and was a mountainous province. They had a very low marshall and their leader had very low military skill and the army itself consisted of 2000 men.
After a few moments of battle, they
WON. My army fled to the neighbouring province, taking a hell of a time to get there, enough for the 2000 men army to take the province and force the disbandment of around 900 of my men. I waited for him to attack again, seeing as I was still in hilly terrain, he did attack again, this time his 2000 men army versus my 5600 men. Once again a short battle and they WON again. My army fled once more and joined with my 800 men detachment. Before I could mount a counterattack they took another province at almost the same time that their northern army took my northern province.
In that moment they had taken all my demesne provinces and my duke died and the game ended.
Clearly I see no other reasoning other than: the AI cheated and cheated badly.
Hence why i'm not playing anymore. It's OK for me to make mistakes but for the AI to destroy what took me what, 5-6 hours to build, no thank you.