How do you beat level 8, "THE HAWKS NEST" on stronghold 2?

First of course you must rescue William by reaching him. Do it as fast as you can before he decided to attack the enemy siege by himself. You might want to bring Ballista and Catapult to clear the enemy forces first before attempting to reach William. Once rescued, don't forget to click him (William) locate and then click the "Return Lord to Civilian Duties" or the "time until defeat" will keep going until you loose even if you bring him back to your castle.
Your next task will be much more simple, build and then attack. In other words, you must first of course build your army and defeat the three lords. Capturing estates will help you a lot in this task. Don't worry, based on my personal experience, they will never attack you even if they have army and also the bridge building by The Hawk won't finish until you defeat the three lords. So, keep concentrate on building your army and don't forget to fortify your castle for the upcoming huge battle with The Hawk and his reinforcement ally, The Bull. Yes, you will get a nice surprise at the final battle from The Bull. One tip, build your defenses and army well before deciding to defeat the three lords, my friend. You won't regret it, believe me.