Spy wars.

I was thinking if anybody has a fool-proof way to sneak in a spy/assasin into a province that has watchtowers and enemy spies/assasins, or is it a gamble in later periods?

no foolproof way to have spies and assassins enter enemy territory. one method, however, to help the odds in your favor is to move several agents through at once. for example, send three new assassins and you level 5 assassin to Constantinople (unknown security) at the same time; security will probably catch one or two of the no-valor assassins, but your more valuable agent should still be alive (at least until next turn)

i tend to dump about 5 spys at first to fin out what thier security is like, and then move my others in after... another way is just to flood them with spys and assasins, and then chaces go your way

I tried that and it looks like there is something to it. Maybe the defending spies/assassins/watchtowers can only catch one agent in a turn then?

I think the same, when I try to kill the pope because he excomunate me, I remember have seen an advice on the screen that combinated plot is most likely to succeed, maybe if you launch your spy/assasin attack from different provinces. It work perfectly, I send 5 assasin from three different province, two were catch but one succeed and finally it cost me 3 assasin (one more died next turn, when trying to escape)

i think its a little costly whenever you want to kill off leaders, but it is definitly worth the effort at times

Killing the Pope too costly???
I find it more bearable to loose 5 spies 100 coins each that have revolts in several provinces worth several thousands...

i didnt say it was too costly but a little costly, im not saying its not worth it either, but generally i accept my excommunicatoin with humility anf fight one, and end the war when my king of pope dies.. take advantage of the fact you have been excommunicated, its not that bad

You brought a very important point out that I overlooked: sometimes the excommunication might help you.
With a strong King and low taxes, you might be able to dispose of that neighbor that was a thorn in your side. To do that though you need to be prepared and have military power to handle revolts and “defenders of Christianity”/crusades. And that means that you are probably winning anyway…