On paper, Shogun 2: Fall of the Samurai reads like the ramblings of an eight year old after his first history lesson, hand stretched tentatively up into the air in class to ask who would win in a fight between a rifleman and a samurai. Before being laughed at by his classmates. Silly child: history has no time machines.
But, evidently, it does have a few time capsules. Feudal Japan was still pretty feudal in the early 1800s, while the British and the French were gallivanting around the globe with steamboats and smooth-bore rifles, and it wasn't until 1842 that the Japanese even allowed Western ships near their borders. But after the West was allowed to dock, Japan went from sword-wielding warriors to regimented platoons of rifle-bearing troops in half a century.
It's not only about the weapons, but also the massive changes to infrastructure that went on during the period. The introduction of railways cut travel times by a previously inconceivable degree, and the assembly line and factories turned Japanese production into a powerhouse. The entire face of the country was altered in a startlingly short time. About as short a time as the typical game of Total War, in fact.
That's not the only convenience of this setting for Creative Assembly. With Empire: Total War already under their belt, they have the knowledge of how to make line infantry work, and the assets in place to bring them over wholesale. Fall of the Samurai even counters the main complaint that people had with Empire: that it was far too big and unwieldy. Instead of the known world, now we have Japan. It's a vastly smaller, more intricate playing field, but it's also one that scales excellently with the game's engine.
None of this is wildly evolving the well-established Total War formula, though. You still build your forces, aggressively expand into neighbouring territories, and then build the infrastructure and economy to fund bigger, better forces. Diplomacy is still a simple affair, with trade agreements and military alliances, and you still dispatch agents to assassinate, sabotage and coerce. Which is fine; no one expects an expansion to rewrite the book. But Fall of the Samurai does rewrite a few chapters.
There's a wonderful sense of imbalance to the whole expansion (which is its own separate install, and playable without the main game), with cannons tearing apart castles that were clearly not meant to withstand cannon fire. The riflemen rip the melee troops to shreds, and the mounted units don't seem to have a response to getting a wall of hot lead blasted in their direction. The economical buildings, too, are all vastly superior if you choose to adopt the Western styles, generating more money for you, and better units and benefits for your people.
All of these are not without cost, however. Each modern building you construct or technology you research adds modernisation to your clan, which makes your people unhappy. Change is never welcome, and when you're dealing with change of this magnitude it breeds a particularly potent resentment. For every shiny new gun, you've got another grumbling traditionalist sowing dissent in your Teahouse.