How do I get the templars to attack my dens?

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.