Ahh... Battle for Wesnoth. The title name is quite deceptive. In this game, you will not only fight a single battle but engage in full blown campaigns of conquest, survival, and adventure. Whether you help a prince to reclaim his throne or you indulge in the study of necromancy to save your people from a worse fate, a game of BfW is played over a series of rounds, in which each player will get in turn to recruit and move units across a hexagonal map to battle and achieve victory.
Generally, the goal in a game of Battle for Wesnoth is to kill the enemy leader, but objectives may vary (a lot even). You will be usually controlling a specific faction (fantasy race) with its own units, advantages, and disadvantages. Elves, for example, fare well when fighting in forests while goblins like hills and mountains.
However, things are not that simple. The day and night cycle can also influence your tactics to a large degree. One day spans six turns. From dawn to the night's second watch, some units receive bonuses and penalties to their attack damage. On top of that, every unit that you recruit will have two different traits that will distinguish it from the rest.
Combat in Battle for Wesnoth is fairly simple to grasp. When attacking an adjacent enemy, a unit typically chooses between two types of weapons, like bow or sword, staff or lightning blast et-caetera. The defender also gets to fight back. Weapons have multiple attacks per combat and the units exchange blows starting with the attacker until all attacks have been spent on both sides.
Units get experience from battle. Once a unit's experience reaches its threshold it will fully heal and advance in rank. Some units can go as high as level 4, while some can choose specific paths when advancing. Leveling is fairly useful in single matches but becomes very important in campaigns.
Battle for Wesnoth has a basic economic system. Units require gold in order to be recruited. The costs vary and the more units you have on the map, the more gold you will lose each turn as upkeep. The only way to alleviate these costs and gather more gold is to control towns. Towns increase your income and relieve some upkeep as well as heal the units that are stationed inside.
A cheaper and more efficient alternative to recruiting is recalling veteran units. Every unit that survived previous engagements within a campaign will be available for a recall to duty. The benefit, of course, is that you can have experienced units accompanying you from mission to mission. Recalling a unit is also marginally cheaper than recruiting a new one. This system promotes attachment to some units and considering that you can also rename them, it allows for a special game experience in which you are grooming an elite group of warriors among the grunts that come and go with each encounter.
The main Battle for Wesnoth campaign is called "Heir to the Throne". However, there are at least sixteen more campaigns of varying difficulty that come with the standard release. You can also shoot matches on custom maps either locally (hot seat and against the AI) or online. As BfW is an open source project there are also hundreds of add-ons that greatly expand Battle for Wesnoth. These are maps, unit and terrain packs as well as alternative campaigns. I think I also saw an RPG campaign in there.
Finally, you can create your own Battle for Wesnoth maps and scenarios. The game includes a professional map editor that gives you access to all the terrain resources as well as to special map logic items.
Battle for Wesnoth is now at least fourteen years old. That means it had a lot of time to mature into a solid game system. Even though the art style has become a bit dated, and most movement animations are still missing, it evokes that late nineties - early noughties charm of games like Disciples or Age of Wonders. Battle for Wesnoth is supported by many fans through the open source development model. So it is bound to get even more polished as time goes by. Being completely free, I recommend it to any fan of tactical battle games.