How to save the mentor

I finally beat one of these by simply stacking nothing but riflemen everywhere I could, with fully upgraded barricades with archers behind them to pick off the wusses so I could build up the rear barricades. They never got through my first barricade until the Greek Fire showed up, and the 20+ riflemen I had managed to take him down. (Fortunately, the barricades held long enough - he was almost out of range of most of my riflemen!)

For winning this, my first non-story Den Defense, I was rewarded with the Assassin Caltrop Unit and also this little thing called "Greek Fire Barricade" that I'm willing to bet will come in handy later. :)

Keep in mind that you'll only be attacked if your Templar Awareness meter hits max and stays there for a given amount of time. Just kill officials and bribe heralds every time you do something to raise it, and you'll never have to worry about being attacked in the first place.

(One thing to note that's a big difference from previous games - renovating property seems to add about 25% to your meter! It's really easy to max out if you're not careful. Also, liberating a den in the first place seems to max it out every time, but you seem to have a few minutes to lower it before you risk an attack.)