The Peninsular Campaign is a great mini-expansion that changes the core game just enough to make it interesting.
The Peninsular Campaign takes that idea and utilizes it best it can which, thankfully, makes the mini-expansion enjoyable and distinct from the core Napoleon:Total War experience.
You can play as either France, Britain or Spain. Sadly Portugal is unplayable. France has to retain it's hold over the bulk of the Spanish peninsular by driving off raiding Spaniards and swaying the populace to her side. The French armies are vast but not enough to police the entire peninsula.
The English and Spanish have to push the French out with an extremely limited force, the English start with only one tiny region and Spain has two. The French are spread thin however and have left their regions vulnerable as a result so early expansion is easy but soon France starts bringing in larger armies that can overwhelm the English or Spanish on the open field so it pays to know which fights are worth having and which are better left for another day. I had to surrender a few regions just because I knew that trying to win would have been fruitless. Some may see merit in hurtling a few militia at a problem and watching them get destroyed but I am not one of them.
The English and Spanish both have to rely on small guerrilla units to cripple the French internally. These small bands of units are best used in raiding French factories, universities, supply docks etc. These guerrilla units also have the great ability to deploy anywhere on the battle field unless it's too close to the enemy. There's few things more enjoyable than running down enemy artillery from a unit of hussars hidden amongst some trees behind the main line.
Another gameplay tweak from the core Napoleon experience is the pro/anti French levels in a particular region, it operates the same as religion from prior Total War experiences so it isn't too outta left field but it's a nice addition from its absence from the core game.
The original music is a great touch as well with lonely Spanish guitars being a staple.
The mini-expansion is a great deal at $10 for what could amount to 50 hours if you play from all three perspectives.