For side by side co-op Koei's Warriors series is very good fun. Bear with me past the first paragraph.
The series is as cheesy as a lump of stilton, and based around historical scenarios with wackiness applied. By wackiness I mean superheroes, magic, manga hair, and people firing laser beams of death out of fans. The voice acting is entirely dire, as in 'Bob-the-cleaner doing voiceovers on a bad day' dire. The writing is atrocious. The music varies from title to title, ranging between 'scrambling for the mute button' and 'I can live with it'. The enemy AI is as smart as a lump of coal on the lower difficulties and it doesn't get that much smarter on the higher levels, just meaner. A lot meaner. On the highest setting you need a levelled up, well equipped character and good skills to survive the easiest battles.
That's the bad. The good is huge battlefield with hundreds of soldiers and other heroes, simple to get into gameplay which reveals some added layers once you get into it, and billions of unlockables. The average Warriors title these days has something like 40 different playable characters and 30 different stages, plus skills to acquire and level, equipable items, 5 levels of weaponry per character, and some smaller side modes featuring things like an endurance test or a board game. There's good scope for character customisation, varying between the different titles.
The battlefields are unlike anything seen in another game. Each side has a set number of heroes placed at set points, and they will enact orders dictated by the 'historical' battle. There are hundreds of peons around the battlefield, sometimes a couple of thousand. Your offer is dropped in at the correct place for him, and off you go. It's up to you what you do then, and the tide of battle will change accordingly. If you rush off ahead and let your allies get beaten up then your side will lose morale and your general might die, causing defeat for you. If you play too defensively you might find yourself making no progress. Mixing the two is often the best strategy, running in to pick off a threatening enemy hero before heading off to another front in order to shore up the lines there.
The games are designed with co-op in mind. Each character's special attack becomes more powerful if you use it close to your friend's character, and if you both trigger yours at the same time it's death to anything which is not you.
Currently on the 360 you have a choice of:
Dynasty Warriors 6
Samurai Warriors 2
Warriors Orochi
Warriors Orochi 2
Samurai Warriors 2: Empires
Dynasty Warriors 5: Empires
Dynasty Warriors 6: Empires
The games with 'empires' in the title are a hybrid between a basic turn based strategy and the standard Warriors battlefield fighting. Simple, dumb, and incredibly good fun. Samurai Warriors is set during the sengoku period and you can take to the field as Oda Nobunaga and other notables. Dynasty Warriors are all set in the epic 'The Romance of the Three Kingdoms', so you use characters like Lu Bu and Cao Cao. The two Orochi games are a fantasy land scenario which brings together characters from the samurai and dynasty series, and I think that Warriors Orochi 1 is probably the best of the non-empires series. Samurai Warriors 2 and Warriors Orochi 1/2 have the most in-depth character growth systems. The Dynasty series tends to be a lot lighter on that aspect.
The Warriors series isn't pretty, it isn't smart, and it's riddled with flaws which appear in iteration after iteration. It is unique, stupidly good fun, and addictive. It says a lot IMO that Koei have been making these games for nearly a decade, and no other company has managed to make a game that's nearly as good. All attempts at imitation have failed.
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