When Mount & Blade came out, it was an ambitious project that did not reach its full potential. The first standalone expansion (Warband) added some key improvements to the formula, but ultimately it felt like a re-release of the first game with just short of enough new content to justify paying full price for it if you already owned Mount & Blade. With Fire & Sword is the breath of... semi-fresh air that the series needed. The essentials of the gameplay remain almost identical (except for the great addition of powerful firearms and some other additions, which force the player to come up with new tactics). However, it is the setting of this game that really sets it apart from the two previous M&B titles.
Based on Henryk Sienkiewicz's historical novel "Ogniem i mieczem", the game pushes us back to a period in real world history instead of fiction. We are taken to mid-17th century Eastern Europe, where the once great Polish-Lithuanian Commonwealth is facing war with multiple of its neighbouring nations. This is where the facts end, as the player can choose to ally himself/herself with any of the real factions included in the game (the 'Polish Commonwealth', the 'Kingdom of Sweden', the 'Cossack Hetmanate', the 'Muscovite Tsardom' or the 'Crimean Khanate') and alter the course of history. Alternatively, like in Warband, you may choose to form your own kingdom or simply remain neutral throughout the duration of the game.
Ultimately, the gameplay of With Fire & Sword is refined to such an extent that it feels like the most well-designed of the first three M&B titles. However, it is still very similar to the previous games and suffers from the same main flaws at the core: lack of significant unique content in terms of story. The cold and calculated nature of the tasks and actions the player is permitted to do in the game may get tedious after a while of playing. If you didn't enjoy M&B, then perhaps you should skip this one unless you really like history. On the other hand, if you enjoyed Mount & Blade then I see no reason why you shouldn't like this game (its unique and unusual setting, which explores a period in history rarely ever mentioned in video games, makes this experience more interesting).