Just as 'The Etten' showed you how to take a fortress, 'The Best Defense' shows you how to keep one. After a little scene setting, you'll go to the ramparts to watch the enemy horde begin its rush.
The best strategy, particularly in this relatively easy siege, is to leap over the wall and seek the enemy captains, easily located by their insignia.
Killing them is fine, but ideally you'll convert them, so they can bolster your side for the continuing siege.
Some of them will probably make it inside the walls before you're done, and that's fine. Just track them down and neutralize them as quickly as possible. When they've all been accounted for, another wave will attack, led by another three captains.
As before, target the enemy captains ruthlessly- they're the linchpin of the assault. If you can, heal up your own troops when they fall, but.... well, recruiting an enemy captain is probably going to get you a higher level flunky than your current guards, particularly if you've got a good Wealth gem attached to your ring. So pick your battles.
Similarly, enemy drakes do a lot of damage, but they're not so good as you at dominating captains. If one does appear, it's better to dominate it or take it off the board completely than to ride it around.
When the attackers are routed, it's time to run back to the keep.
There's a final post-victory cutscene that doesn't end so well for you.
Still, it's enough to earn your reward and mission completion.