I'll make an addition to that. Good mobs might let good players pass unmolested eventually.
As far as forges in the castle... The Philo Stone requires killing Uyp, Vicar, Geber and the hound for cps. The Thundery Gaunts require killing the smith, Wu, Uyp and hound. So if you're forging, you're a past offender or bought bloody cps.
A couple quests pass through the area, but its a fair bet that at _least_ half of the people that undertake it have killed in the castle at some point.
The only really genuine way to code it would be to see if you have received first kill xp for being in the castle. If no, then let you pass. After that, attack on sight. All your other arguments are completely bogus.