I have given up several provinces and reinforced my border with stone walls. I use the sally-quickload bug (you can sally as often as you like) and decimate the enemy siege army when it is vastly larger than the garrison force. Their horses don't stand a chance when subjected to oil and arrows and stones from above and facing a wall of spears to keep them at bay.
Any day now I will start conquering back my provinces and kill them all! :D