Help was asked on the CC2-I Yahoo group about the arrow slit tools and I was unable to upload the following visual help there so I post it here, hoping it will be usefull to many...
Note the insertion point -first point to select- is on the inner wall, at the middle of the break. The second point is anywhere on the outer wall.
If you don't want the recess (alcove, embrasure) just answer 0 at the first prompt (alcove depth).