Don't know if people have beaten me to it, but Den Defense is fun, and it's fine and really easy, and you'll always get the best reward so long as the Den itself takes no damage, so its a win-win for EXP, quick cash, quick level ups, etc. You can avoid Mediterranean Defense entirely by just doing this, but your wallet will definitely suffer.
Have one Air Assassin, as soon as you get them, in between two Riflemen where ever you get a group set up. Don't bother with Crossbowmen once things start becoming available, Riflemen are just fine. In fact, don't bother with Crossbowmen ever, I never used them after the second Den Defense, Air Assassins are much better. Don't place more than one air assassin in a single group, they may only hit one or nothing, one AA is just fine.
Splinter bombs are good in only certain situations, but are easily overshadowed by Ezio's gun spam and the Air Assassins and their double blades so you can save on Morale. Use your discretion. As soon as possible, get your weapon barricade up (Cannon is the last and best) and upgrade it to max and if things start getting hectic, place another barricade to upgrade to max. Always be aware of the wave timer and watch for Stalkers. Only place Bombers on a barricade, it's the only place they will be able to reliably do damage in the case of enemies getting too close. Don't mistake them for being strong either, as their bombs don't do anything against non-flesh targets.
Never place your Assassins too far, like on the edge of the screen, or you won't even be able to kill the Stalkers if they come and they will really start tearing into them. You will lose points this way too and just waste Morale.
Scrape your cursor over the dead bodies constantly and mash Circle to grab loot, you can get Morale from this.
Victory will be yours. Always.
...Unless you don't have enough riflemen to destroy the war machines.