yes in the castle level you need to stand by the wall with a small square hole in it...facing the two story red japanese building that has the sniper room in the top right hand corner of your screen. now while facing the wall you will notice that the wall runs into a building creating a corner. start in that corner and sprint run back and forth from the hole in the wall back to the corner, back and forth, back and forth. eventually you drop under the level. you can kill and be killed but you can see them and they cant see you.
stevelknieval
aka surfinsteve