From the Avalanche Press website:
The Gunpowder Strategy games from Avalanche Press Ltd. cover regional and global conflicts from 1700's and early 1800's. These are multi-player games, driven by card play and often filled with back-stabbing diplomacy.
Each player has armies and fleets. There are also a handful of generals and admirals to assist these in battle and in movement.
Players have allies; some are set at the beginning of the game, others alliances are forged during the course of play. Everyone else is an enemy. No one is “neutral”; those are just enemies you haven’t attacked yet.
The map is divided into land areas and sea zones; movement is point-to-point. Armies move on land, fleets at sea. Each land area is rated for the amount of money and manpower it generates each turn. Manpower represents not just fresh recruits for your forces, but also the things made with human labor: food, uniforms, weapons and so on. Money is, well, money. You expend manpower to rebuild your forces, and money to finance their actions. Thus you need to hang on to areas that generate these resources for you, and take them from the other guy.
You do that by defeating enemy armies, and besieging enemy areas. Combat is conducted by rolling dice, one for each attack factor. These hits must be sustained by enemy armies by reducing them in strength, or eliminating them. A good general lets you roll more dice. Each area is rated for its garrison strength; to capture it, you have to defeat the garrison troops through siege (in addition to driving off any enemy armies there). The procedure here is very similar.
Throwing a twist into all of this is card play. The universe is loaded with random elements; life does not unfold as an orderly series of “phases.” After a short countdown to start the game, players can play cards at any time, in any order.
The cards are the centerpiece of the game, giving it its free-wheeling nature. Generals can vacillate. Mercenaries can be hired. Local militia can appear to help. Soldiers might run off to loot. Bridges get burned, snow falls early, the harvests are bad, the harvests are good. Royal marriages, minor country alliances, the rise of new leaders — all sorts of events take place during card play.
Gunpowder Strategy page at Avalanche Press