We tried out a scenario utilising defendable cover last night. The defender gains an extra Oran stone just like shooting, using the same method keeps things simple.
The rule seemed to work really well but we did have to introduce a zone of control rule or enemy models could easily bypass defenders which would defeat the objective of having defensible terrain. We kept it simple and just said that an enemy model could not cross the wall if within 1" of a defender.
We imagined the wall not being there when working which models were in B2B contact which meant that a maximum of 2 attacking models could be in B2B with 1 defender.
The next conundrum we faced was when the defenders wanted to pull back from the wall. Breaking of allows for a Cautious move and in theory, the attacker would move the same distance for crossing an obstacle. But instead of the models ending in contact again (as if they were in difficult ground) we still took the width of the wall into the equation so the attacker would fall short by 10mm or so. We think this is reasonable and realistic allowing defenders to break away.
All in all it worked really well, even if it did give some nice targets for the Delgon area weapons. In our game the attacking Delgon just about achieved their objective (take two points within the compound) before the reliving knights wiped them out. The Delgon were just one turn away from losing. Here's a couple of pics:
Delgon closing in. At the end of the game