The computer cheats!

[Ludens]

The Pope tends to tolerate small Catholic Factions attacking large ones, but not the other way round. This is indeed carried to the point of madness when it comes to reconquering lost ground or attacking in sea battles, but there it is. It is not the only way the computer cheats.

However, I have never seen cheating naval invasions. I don't know how BKB's map looks, but if it is like the vanilla M:TW map, one ship in the Gulf of Lions ought to have prevented it. They could have used a line of ships along North-Africa to invade.

[King Henry V]

The seas in BKB are exactly the same as in vanilla, and the next trun I checked to see wether they did indeed have any ships along North Africa, but they only had one of Algeria.
Some new updates:
I can't launch a Crusade against Constantinople, because the Byzantines are allies of the Pope. I have no allies, even those who are not allied with my enemies do not want to ally themsleves with me. Then unexpectidly, without even being excommunicated, the Pope sent its troops to attack me in Naples! I managed to beat them off (I thought for a moment wether even resisting His Holiness would get excommed), but decided not to invade the Papal States and Rome as the last thing I need now are a bunch of angry, devout peasants revolting in Brittany and Anjou. Fortunately, I have managed to build a fleet of caravels (only saving them by sending them into the Central Med, where the Byzantine ships can't get me) and pretty much destroyed the Med fleet of the Byzantine Navy. The Danes, on the other hand, still blockade my Channel ports.

[Dutch_guy]

I have no allies, even those who are not allied with my enemies do not want to ally themsleves with me. Then unexpectidly, without even being excommunicated, the Pope sent its troops to attack me in Naples!

yeah that happens when you get to big, a feature which I just hate.
Doesn't really makes sense , don't you want to get allied to the most pwerfull faction in the game ?

The Pope once attacked me in Tuscany , when playing the Italians ( high )
I was lucky to beat them in Tuscany and also lucky that the Pope also held Naples, so I could get the Papel States and Rome and still fight a defensive war against the Pope in Naples If I would want to.

The best part however was that my very,very bad king died of an illness the turn after, so Excom was lifted, and was replaced by a great heir , who I had trained against the Eggies from infancy

[King Henry V]

OK, I was powerful (largest number of troops in the game), but if say Spain or France decided to trun against me, my empire would have a hard fight on its hands.

[King Kurt]

This is frustrating when this happens. In my current Early English campaign, I am the largest by some way. I have conquered all of Islam single handed, pushed Byzantium into 2 small Islands and have been waging war against the Novos for ages. I haven't been near the Hungarians, but they attacked 1 province of mine - Moldova I think - I lift the seige and I am currently under threat of excomm - ther is no justice!! Nobody wants to ally with me either. I have even given the Pope Burgandy after he bribed a rebel army to his side.

[EatYerGreens]

I lift the seige and I am currently under threat of excomm - ther is no justice!!

When your forces are inside the castle, note that the province around it has changed colour to the invading faction. It's now their land, so your siege-lift, in fact, counts as an attack on them!

Pope says "get out within 2 years" but, by winning the province back, you've achieved that bit - it's not their land any more. Now all you have to do is abide by the "no further attacks for 10 years" bit and all will be okay.

The purpose of the warning is to give you the choice of politely pulling out and handing the lands back, or pressing on with ending the siege inside 2 years - e.g. one year of attrition and assault in year 2 - but make sure you win that one, or else!

What really gets peoples' goat is where some other faction starts the war but it's you which gets given the warning, as per what happend to KHV.