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I don't think a write up of Seven Samurai brings anything to the table that isn't put there by saying Seven Samurai, unless you were going to iron out some details, toss in some new character profiles and give some time lines for a GM to produce a fun retelling of seven samurai.
I like the idea of rallying a village to help defend it. In the TV mini-series, Pillars of the Earth (and novel, though never read it.), there is a battle of the correct scale for PCS' - the attackers are about 20 men at arms and knights attacking a small town. A small number of 'PCs' help rally the defense with a nice small scale battle.
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For a twist: The villagers knew they could not win and poisoned their grain. The bandits have taken and eaten some of it, but not before selling off a great deal to various local populations. During a time of bad harvest can the PCs manage to find and stop the shipments before they are consumed? Should they cure the bandits if they come across them dying?
Honestly, just the characterization of Tymas allows this some life. Although the actual plot is a great way of getting players to hate a location, Tymas could very well develope into an NPC, fighting for the player's help in ending the pressgang-like activies of the guard.
So from the other comments we have established that Tymas' perspective is not the one we want the PCs to share for a number of reasons. Perhaps the players could be inside the city on some sort of espionage mission and gradually come to know or befriend him and learn that most of the city guard has been pressed into service. That would then make assaulting the city much more of a moral dilemma.