All good answers! The main point here is not the effects of the nuclear war since it'd be so far in the past that it'd be mostly myth and legend.

I hadn't considered an asteroid or a plague, both of those might actually work better. The main reason that so much technology is lost is the conventional warfare that comes after the cataclysmic event and with a plague I can see a massive war over medicine and technology ending with only the few people that were naturally immune surviving and having to rebuild society.

However, I also want some technological holdouts(A moon colony for late game adventuring actually) and most of the gun-powder and Iron-clad ships would be from or before the event.. So then 2-300 years after the event? That would also leave most cities overrun by nature or even overgrown compleley right?