I've been mulling over this particular area of history recently, and it occurred to me how little I really know. My biggest question is, of course, what went wrong?
I understand that the invasion of Spain was disastrous if nothing else, and ultimately led to a massive diversion of troops (or something along those lines) that were better used elsewhere. The Russian Campaign of 1812 is also something I've come to understand as a huge hole in Napolean's planning.
But I don't understand the reasoning behind them very well. I get that the Spanish Bourbons(?) were not compliant with Napolean's will, so he followed through with his threats of invasion. Can they be compliant? Can Napolean get by without installing his brother as King of Spain? You may have already realised that I'm looking for a way for the empire to survive (not wanked out, but there and with more territory than it was left with in OTL).