So I finally have a some time to sit down and respond this, sorry it took so long.
Also for clarification as to using the word exploit. An exploit is something that is not intended, which is achieved by doing something unexpected. Exploiting a mechanic is not an exploit. So castle stacking for example is exploiting the mechanic of capital bonus and marshalls. Building extra castles - more than typically allowed - is exploiting a bug to achieve an unexpected outcome, thereby cheating. The term gamey was first introduced on these forums to refer to the first case, but has since been hijacked by people using the later.
Well that is a good clarification on what you mean by exploit.
I think the castle stacking via build event is a good example of an exploit that needs fixing. It allows you to get around the one city and church must be built rule which seems to be very against intent of system rules. The cheep price I can live with as it wont always succeed but it could stand to be raised a little maybe. *shrug*
I have only seen you bring up the castle issue is their any other exploit that seems to be glaring real bad in the game? As the thread title implies multiple exploits. I haven't had enough time to play ck II vanilla since way of life came out so I am a little behind in what exactly is going on with it issues wise as I have been playing a total conversion mod on an older version.