Can I withdraw an assassin from a city?

I was playing around with the Mediterranean Defense in Assassin's Creed: Revelations and ended up assigning one of my den masters to an assassin-controlled city.

I didn't actually want to go through with that, but I couldn't find a way to cancel the assignment. Even pressing "Back" just popped up a vague confirmation that turned out to mean "you will assign the assassin to this city if you choose to proceed".

So now I'm short an assassin and have a den without a leader. Is there a way for me to get that assassin back from the city I assigned him to?

(Assuming you have others who aren't out on missions,) you can swap in another assassin for that one. This is square on the PS3, not sure about 360/PC but it should show in the controls at the bottom. You can do the same with empty slots for any assassins past the first, but I have yet to find any way to remove your last assassin from a city.

Note that stationing your den leaders elsewhere doesn't actually appear to affect your den lock status.

Once you have assigned an assassin to a city, that city must have an assassin assigned to it at all times. So unless you don't have another assassin available to switch with, that one is stuck in the city.

If you have more than one assassin assigned to the city. You may remove one of them from the city provided you don't already have all your assassin slots filled. To do that, just "swap" a vacant spot that you have with the assassin you wish to remove. Not very intuitive I know but that's how it is.

Putting your assassins into a city is a great way to clear up some room in your lineup so you can keep recruiting more assassins. They can even be assigned missions at that city so you don't have to use up your assassins. Don't overlook this feature, constantly sending your assassins on missions is a great source of quicker income with say ~1000 Akçe per ~4 minute mission (and they go by fast) compared to say ~20k Akçe per 20 minutes. If you have enough strong assassins, you could be constantly earning money with very little effort at all, just a bit of your time.