Castle instance, similarly to fortress instance is a great way of obtaining full drops like A-S grade weapons/armors and enchant scrolls, plus lots of KE.
To enter the castle instance 2 requirements must be met:
1. You must be in a clan that has a castle (duh!).
2. Your clan must have a contract with another clan which owns a fortress.
Once you got your party ready, get to the castle and go to the lower level dungeon where the wh is located. There you will find Warden NPC, from which u can enter the instance.
After entering you got few minutes to organize and buff before the first raid spawns.
The first one is always easy, its not good to do it on high levels, but it still gives KE even on dark blue.
The next one does a lot of stun, so shock resist is recommended.
For some reason it took a liking to the healers...
The 3rd one is the hardest one. Hits hard and has a lot of HP, but the drops are pretty good.
It's important to have a tank, but it's not necessary if the group is ranged, whoever is aggroed can just run around.
Castle instance resets every 4 hours, so you can farm it in that period of time. The bosses drop from 200-500 KE and at the end you get quest items (140-150) which you can exchange for KE.
All and all from 1 castle instance run your whole party gets around 1400 KE. Plus the full drops if you are lucky enough.