first off assassins .. there should be more info about what you can use them for (in civilpedia) tok me like 4 games to figure it out (only played 4 games :))

So i have moved em to enemy cities and umm "spaced" 2 turns .. tho i read someone said you can fortify em ? Sometimes i failed .. sometimes the assassin succeded and disapeared .. sometimes it was back in my capital to be used again (no idea if its meant to work like that , because of lack of info).

Ok so now i have 2 of my own assassins (1 newly produced) , and 1 enemy assassin in my capital city (I do NOT have open border treaty with this enemy, tho im not at war) i saved at this point and well .. tried to reload hehe , but every time the assassin bring my capital to rebel ... Is there no way to deal with a Assassin??? am i just to accept that i cant defend myself ?

You can Defend, but that is hard.
A Low lvl Assassin witch has only +3xp from varrack can be stopped (95% i say) by 3 defending units.
Mid lvl Assassin witch has +3xp from barracks and +2 from militaristic/great general specialist needs 6 or more defending units.
High lvl Assassin witch is atleast lvl 3 (barrabck + militaristic + feudalism + theocracy + great general specialist + xp gained by him) Cannot be stopped by units unless u have sick stacks like AI does...

Another thing. To get very high chance of stopping enemy assassin you need assassin in the city that is currently being assassinataed (before assassin activates). I dont know the chances in numbers, but the bigger the differance in expiriance betwean assassins the higher chance the more expirianced has.
For instance. Assassin lvl 50 can withstand all Assassins with not cheated XP :)

Also Police State govermental Civic give you some percentage bonus vs Assassins.

As for assassin dissapeariance. If assassin Fails or Succeeds, he can teleport back to your capitol or die. Dont know how it is actalya used, but i say its this way:
The chances depends on his XP as well as Enemy bonus vs Assassins. The bigger differance your Assassin have over Enemy bonus the higher the chance.
But remember that it may be static chances (eg 25% chance of survival) that no1 can change.

As for Civopedia, yes it should get updated.

Hmm so a 10 exp assassin is a unstopable force (8 exp as charasmatic leader)? (im guessing you mean lvl 4 )

Shouldnt a assassin be able to kill enemy assassins (within your own borders) ?

This kinda ruin the game imho .. 3-4 assassins can bring a civ down. In the game i played i got 6 turns of resistance .. the city had 8000 culture so i lost a huge area of influence (ok i know its temporary but still). If the AI had sent 3-4 into my 3-4 largest cities and "set them off" at the same time ... and attacked at that time. I doubt i could have done anything about it.

I'll play some more games , but i seem to get very much ahead of the AI .. and its no fun at that point so i start a new game .. ok i havent tried playing as the celts hehe.

Ever heared about decrearing war ?
If he is in war with you he cant have any of his units inside your units/cities. Spy i belevie is unique here.