Quoth Didz

Probably not a good idea. The ports would be a serious defensive weakness and given that there are very good reasons for not having level ground in front of a castles walls it would not provide much benefit vis-a-vis the dead ground issue.
Please have a look here:

http://www.castlewales.com/rag_tr13.html

If you study the photos (they may take a while to load) you will notice ground level gun ports.

Defensively they were no more a weakness than arrow slits. Not a great arc of fire admittedly but handy for the odd shot at attacking forces.