If you have ever wondered how Jesus defeated Satan on the cross then you could do worse than listen to Sinclair Ferguson's lecture on "Christus Victor" at the Highland Theological college. You can find it on this page. In it he gives a superb defence of the penal substitutionary view of the atonement.
You need to be prepared, though. This is an academic lecture, and you may need to listen two or three times to get all of it, but I think it is well worth the investment of time.