Another good stack defender is to get Crossbowmen(Or Arthendain if you're RoK) and promote them with Drill IV, Both Defensive Strike promotions and Blitz. Since the AI is rarely smart enough to use Blur, you'll often take out 50-60% of every attacking unit's HP. If you have another powerful unit with Drill IV to defend, a 1,000 unit stack won't even hurt you.
If getting Crossbowmen is unreasonable, Longbowmen will be almost as good(though with only a 95% hit rate instead of a 100% hit rate) and you can have multiples.