PLACEMENT-CASTING

Really bad placement adversely affects casting. If someone has six things attacking them it is more likely that their casting will get disrupted. The formula is:

+ spell skill%
+ placement * 10%  (up to a maximum of -100%)
+ your Combat Casting skill % * 0.3

For example, a mage who tries to cast magic missile and knows the spell at 90%, and who has a placement of -4, would have a 50% chance of casting the spell. If the mage knew Combat Casting at 100%, the mage's chance of success would be increased to 80%. (+90% -4*10% +100%*0.3 = 80%).

Note that Combat Casting will only help overcome a negative placement. It will not help a positive placement. Also, if you keep your skill percentages maxed and learn Combat Casting, you will always have at least a spell skill % - 100% + 30% chance, or about a 20-25% chance of casting even if your character is totally swarmed.

You can also get your pets and melee friends to form a wall around you to make casting even safer.