I have captured every den in the city and one of the assassins challenges is to win three den defences. I noticed this at DNA sequence 3. I am now at DNA sequence six and they havent even attacked one of them. Please help me.
In order for the Templars to attack your dens, you have to have your Templar Awareness maxed. Once it hits max, any further attempts to increase your Templar Awareness will provoke an attack on your den. So, if you're maxed out and you kill a guard, they will attack your den. However, they will not attack your dens if you have an assassin assigned to it. So, if you've already assigned an assassin to all of your dens, you won't be able to play Den Defense again without replaying the original Den Defense memory.
In order for for one of your dens to be attacked, your templar awareness needs to be maxed out. If you renovate a shop or get in a fight with guards while the awareness meter is full, it will cause an attack on one of your dens. Note that if a den has a den leader then that particular den will NOT be attacked.
So if you want to start a den defense mission, don't start assigning den leaders to dens just yet.
If you want one of your dens to be attacked, maximize your notoriety then keep committing mass genocide until your one of your dens gets attacked. If you have Assassins assigned to your den, move them to Assassin-controlled cities using the Mediterranean Defense option and selecting a city to move them to.
If you somehow manage to build every Assassin Den, liberate all 14 Mediterranean cities, recruit 82 rebellious citizens and train them ALL to the rank of Master Assassin, then you can never play Den Defense ever. But, by now once you've played too many rounds to the point where it gets too hard and you get 100% synchronization, it's safe to say no one has done that yet. Hopefully.