I'm here to give a different perspective than "just play Crusader Kings II instead". It's true one can see the prototype of Crusader Kings II in Sengoku. They share many game rules, Crusader Kings II having more stuff in it and being the bigger game in every way. In this review, I'll focus on why one might want to play Sengoku over its bigger successor. Sengoku is very focused. The game is less a story generation engine and more a wargame. There aren't hundreds of small game systems that come together to generate an epic story of your family line. Instead, there is lots of war and strategic decisions about how to conduct it. It's practically impossible to make progress towards victory without waging war, and waging war is what you'll do a lot in the game. You'll still engage in some diplomacy, but in the end you do it to either gain an ally who will wage war on your side against your enemy, or to avoid waging war against someone specific you want to avoid - for now, while you deal with other enemies elsewhere. Even during peace time, you are already thinking about the next war. When one war ends, there is a short lull when you regain your strength and resources, scope out where and when exactly start hostilities next, and pick out who do you need woo to be on your side when the time comes. Really for the game to be so focused on one thing, it could put some more spice in it, for variety. There are even little too few systems to engage with, and at times the game gets repetitive. Ways to gain allies and make enemies are not very numerous, and it feels like your opportunities to affect the political environment are too limited. After identifying who you want to be on your side, you can activate the three or four things that increase their opinion of you, and if the opinion score does not get high enough then it does not. What saves the game from total monotony is how the changing situation on the map over the course of a game needs to be adapted to. It's different to be a small clan dealing with many small neighbours, than it is to be that same small clan a little later when a big rival has swallowed all those small neighbours you used to have. Likewise once you control more and more of the map, the approach you'll take changes, as your opportunities and risks on the map are different. The other side of the war waging coin is internal diplomacy. No one ruler can personally control a very large territory, you must delegate. The bigger your domain gets, the more people under you you'll need to keep things running smoothly. If you don't keep your underlings happy, that will surely throw a spatula in the wheels of you war machine. Your subjects can declare independence or join your enemy, persuaded by an opposing clan's diplomatic efforts, the same way you can and will try to cause internal conflicts within your opponents ranks. Best way to keep people happy is to conquer more territory for them, but war is expensive and also causes unhappiness when prolonged. The game asks you to balance your resources between conflicting needs, chiefly the demands the external threat posed by other clans presents, and the requirements posed on you by your clan's internal politics and the risk of your clan members rebelling. You can only succeed by choosing carefully where to place your bets, what to neglect and for how long. The game can get monotonous at times, but I really like it, and do think it'll over time offer me more enjoyment than Crusader Kings II.
