Battle for Wesnoth (I'll just call it Wesnoth) is a solid game. I've been playing this pretty much ever since it first came out, and I've enjoyed it the whole time. It has fun, detailed gameplay, a bunch of content, some surprisingly cool music, and even mixes in some RPG-esque elements.
First is the gameplay. I am continually amazed at how complex yet approachable this game is. So, most of the game revolves around the terrain; it affects movement rate and chance to hit, which are pretty much the basics of the combat. Additionally, there is a day/night system that affects unit's attack damage and some special abilities. Chaotic units do more damage at night and less during the day, Lawful units do the opposite, and Neutral units are unaffected. combat is split up into one of two sections, Melee and Ranged. These don't actually have anything to do with the distance units attack from, but instead determine which combat values to use (if any). Units have two sections to each combat value: damage and number of attacks. Each individual unit has health points (no duh), but they also have an experience value and two traits. I'll get to the experience next, but for now, the traits are two modifiers determined by race. Every unit gets their own when they're recruited. Some units, like the Undead, don't get traits, while others, like the Elves and Dwarves, can get traits others cannot (like Dexterity). The traits, whose effects range from doing a little extra damage in melee to getting an extra movement point, are a pretty cool feature. Lastly, specific units have different resistances to different damage types (e.g. the Woses, who are basically Ents, are especially vulnerable to fire). The game is hex-based.
So, seems pretty straightforward, right? What is surprising, though, is the level of complexity it allows for. Every individual unit type has its own terrain values for how much movement passing through them costs and how much protection they afford. This, combined with the many types of terrain and the hex map, makes for some very strategic gameplay. The goal is to stay in the terrain advantageous for you and keep enemies away from their beneficial terrain types, while still moving forward and stopping the enemy from getting behind your lines. The thing is that there is usually no set way to do this. You can take the cautious approach, stay in cover, and engage only on your own terms. However, typically speaking, the better protection terrain provides, the slower you move through it. Since games have a set number of turns, you have to be plan ahead, or be willing to give up crucial units or positions. Additionally, units regain health slowly. Indeed, if there isn't a village or a healer nearby, than anyone surrounded is pretty much screwed. Thus, determining which units you do and don't need is also necessary. And lastly, keeping track of the day/night cycle is important as well. If you ignore, you'll soon find your enemies killing you in a few hits while your units do less and less. And finally, you have to keep track of which units to the most damage to each other, and take the least damage back. You may want to use your Heavy Guards against Undead units, because they have Impact attacks which deal extra damage to the skeletons. On the other hand, you may want to use a ranged unit to attack them, because Skeleton Warriors have no ranged attack and therefore can't retaliate. While not especially difficult to get into, the gameplay in Battle for Wesnoth is very complex, and there's a great level of detail.
Next is the units, who are also pleasantly complex. Almost every unit has multiple "levels," which they attain by gaining experience (which is gotten from killing enemies). Basically, the higher the level, the better (and usually cooler) the unit is. They get more damage, more attacks, more health, and a larger amount of experience needed to level up again. They also get a new sprite and unit description (every unit has its own description page giving background on that type of unit. A nice touch, worthy of mention). There are also multiple upgrade paths for units to take. Peasants, for example, can become Bowmen or Spearmen when they level up, and Mages can become Red Mages (combat types) or White Mages (healer types). This is quite possibly my favorite thing about the game, because it gets you really attached to your individual units. Also, it means that you have to use your units carefully. Leveling units up is a long process, and while you'd be a fool to not utilize their abilities, you also need to protect them, because it will take a very long time to replace them.
Next let's talk about the visuals. The graphics in Wesnoth have evolved over the years, and frankly look pretty great at this point. Let's start with the basics. the game is sprite-based; no 3D graphics here. Units have idle animations, multiple death animations (you kill a Wose with swords it falls over, you kill it with fire, it burns up), and different attack animations. The only thing they're missing is a movement animation, but you'll stop noticing after a while. While you won't be astounded, you probably won't be retching either, and you may even find something cool the first few times. Probably the best thing about the visuals, though, is the art. The different terrain types blend together amazingly well. You never would have known that lava and grass could be right next to each other so naturally. The edges blend together and merge, and the result is one of the nicest looking map generators you'll ever find in a hex-based game. While that seems like a very bold statement, the reason I say it is because it's true, mostly for how well hexes of different (and also the same) terrain types look right next each other. Granted, the visuals are nothing to lose breath over, but they are extremely functional, pretty good looking (everybody knows sprites are sexy), and aesthetically appealing.
Next is the sound. The music is Wesnoth is, surprisingly enough, done almost entirely by users of the game. Does the fact it's made by fans mean it's inferior? No, not in the least. The tracks are done with classical instruments and sounds, so expect to hear a lot of strings and drums. It fits the medieval fantasy theme of the game well, and is actually pretty nice to listen to on its own. That said, there are a few tracks in there, especially one from the first version of the game, that are...less pleasant to hear, but they are usually not bad enough for you to dislike them (and if you do, you can just turn the music off for a sec). So despite a few flukes here and there, the music is overall quite nice. The problem with it is that there is no way of skipping or selecting tracks. I mean, I'm not asking for a music manager like the Age of Wonders games have, but some way of managing or at least skipping the currently playing track would be nice. This is usually less of a problem during campaigns (the heart of the game), because most missions have set tracks that fit the mood well enough, and no others will get in. For single scenarios, though, it can just be a little annoying to be playing and then have a track you hate pop up. Next is the sound effects. Well, first off, the sound effects are fairly complete. Different attacks have different sounds, different units have different death/damage cries, and so on. Much like with the lack of a movement animation, there is also the lack of a movement sound. This isn't really a problem, it's simply the most noticeable part of the game that doesn't have a sound effect. As for the quality of the effects themselves, they are above average, though they're also nothing amazing. Some of the unit cries sound almost a little goofy, but the attack sound effects are solid, with the melee attacks sounding like melee attacks, the magic attacks sounding like magic attacks, etc. Overall, good sound.
The next section is on the balance of the game. First it hitting and getting hit by the enemy. Wesnoth uses a hit percentage calculator which the game makers assure is not weighted towards anyone. You won't believe them at first. There will be times when you hit a particularly unlucky streak and feel like the game is intentionally making you miss. When you see that there are times when the enemy also hits unlucky streaks, however, you start to feel a little less cheated. Additionally, since results are randomized every time (unlike, say, Fire Emblem), you can reload your last save (the game autosaves at the beginning or end of every turn, depending on your version) and simply try again. It won't get you out of any particularly sticky situations, so you can't really cheat the game (no, your wizard with 1 health who's surrounded by enemies probably won't survive no matter how many times you reload), but if you feel that both of your units missing all 4 attacks was poor luck, then you can try again. Though it can still be frustrating from time to time, the game is pretty well balanced in regards to hitting. The other section is faction balance. It surprises me how equally the different playable factions are matched. I always feel like some race or group of units has a clear advantage, but when they start fighting, they're usually able to both stand their ground. Granted, there are a few factions (like the elves) that are slightly better than the others (especially on heavily forested maps), but not so much that they have an unfair advantage (unless that is intended by the mission).
Finally, let's look at game content. Wesnoth is really pretty full of things to play. The game comes with 16 campaigns, more maps than I care to count, and multiplayer functionality. There are network, online, and hotseat options. I've only tried hotseat before, but it works very well with two or three people (so that there isn't too much waiting), and from what I've heard the online works well too. Additionally, the game comes with a great map editor accessible from the main menu which works like a charm. Unfortunately, you can't make scripted scenarios without getting and modifying the source code version of the game, but several others have already done so. That is something worthy of mention in Battle for Wesnoth; there is lots of user content. Custom races, maps, campaigns, and everything else are all available online, so you won't be bored (assuming you can access it, unlike me). Even if you aren't a programmer, though, the map editor alone is enough to tide you over for some time. Fantastically easy to use, the map editor also gives you something that pre-created maps can't; the satisfaction of playing on a map you made yourself. It may not sound like much, but being able to create a quality map (which is made pretty easy by the editor itself) and play on it is one of the most satisfying things to do in gaming.
Now, I know that I started the last section with "finally," but there's one other aspect of the game I need to mention. Battle for Wesnoth is 100% free. Yes, you heard me correctly, it's FREE. Now, it would be kind of impossible to be able to view the quality of the game in context without recognizing that it's something people have been doing as a hobby. If there was one free game I could pick to overturn the misconception that all free games are crappy, it would be Battle for Wesnoth. It has as much, if not more, complex mechanics and content as any number of "blockbuster titles," and incorporates features (such as the unit upgrades) that have yet to be done so well elsewhere. It needed to be mentioned, and while it is not what makes the game good, it does certainly serve to make the game more desirable.
Battle for Wesnoth is a great game. With easy to learn but amazingly complex mechanics, a great unit system, and tons of content, this game is worthy of your attention.
Gameplay: This is the real draw of Wesnoth. The game is fairly easy to get the hang of, but is extremely deep and complex at the same time. Strategy fanatics shouldn't be put off by how simple it seems at first, and laypeople to the genre shouldn't avoid it because it's complicated. It may not take you a lifetime to master Battle for Wesnoth, but the point is that Wesnoth is equally enjoyable by an experienced player and a new one alike. Overall, great gameplay.
Visuals: Nice looking sprites, great looking maps, and some nice extras like idle animations. What's not to like?
Sound: Good music and complete effects are the draws here. The music (mostly) fits the mood and the sound effects are pretty good, so there you have it.
Game Content: There are 16 campaigns, most between 10 and 20 missions long, and you may even want to play a few of them multiple times. Throw in multiple difficulty levels for the campaigns, more single scenario maps than you can count, a great map editor, and some nice multiplayer options, and you get a happy gamer. Quite strong in this department.
Overall: Battle for Wesnoth is complex and fun to play. I didn't include that it is free in my rating, but you can't ignore that fact either. Really, once that factors in, you should have no good reason to not get this game, unless you really hate TBS games. I strongly recommend this game.
You can get the game from its site here: http://wesnoth.org/
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All screenshots taken from Wesnoth.org