I've played this at a con last weekend, and it was terrific.
At first you are gathered in the city, then the abbyss spews forth the invasion, and action commences.
While we were rescuing people, fighting off demonic invaders, and avoiding traps, the repeated improvement or worsening of the situation in the several districts (and just outside of the walls) has you eager to finish your encounter and defend the city elsewhere.
There is chance to have your skills tested, not limited to social and knowledge rolls, though those of course come in handy.
Defending the battlements with a freaking large ballista is awesome, all the while defending against enemies coming from outside and within.
Though this will work in PBP, like previous specials, this one will truly shine in a F2F Multi tables game.
All in all, this is a must play, imo.
There is something to be said, about defending a city against demonic onslaught. Awesome comes into the direction...