Brute Wars is a tough game to categorize. It's part RPG and part turn-based strategy, I suppose. The result is not a bad game, but it's very simple -- it's much less complex than your typical example of either genre, and consequently you'll find it doesn't have quite enough depth to be able to sustain your interest all the way through.
So, the basic principle of Beast Wars is pretty simple. You move along an overland map consisting of a number of linked circles. Most points are simply combat, but a few circles have item shops (some which offer basic healing items, and some which offer various upgrades), and each screen has one castle fight. Once a combat circle has been cleared, moving back onto it is still not entirely safe -- there's still a chance (although not 100%) that you'll be thrown into another fight if you try to move back through that circle, so you can never move around the map completely freely.
Fights are quite straightforward. You have six creatures arrayed in a three-wide, two-deep formation, and your enemy has the same. Each creature is characterized by five properties -- its HP, its power (the amount of damage it does per attack), its counter (the amount of damage it does to enemies who attack it), the number of actions per turn it has, and its attack range. A creature can either attack or switch places with another creature using an action. Creatures with only one action per turn, then, are less flexible since they can't both move and attack (at the outset of the battle, of course, there'll be plenty of creatures to attack, but as the battle progresses and creatures get knocked out, you'll have to move them around), but their attack power tends to be stronger appropriately to compensate. The attack range is probably the trickiest point -- some can only attack the creature directly ahead of them, others can attack all three creatures in front of them, and some can attack two spaces away (useful for your back row). Some creatures even attack randomly. There's also a magic system -- as the battle progresses, you gain magic points, which you can then later use for spells. Most battles don't even last long enough for you to accumulate enough points to use the most powerful spells (and the points don't keep from battle to battle), so I ended up not using the magic very much (just an occasional heal).
At the end of battle (assuming you win), creatures that survived gain a level, while creatures that got knocked out lose a level. This provides an irritating kind of feedback -- your weak creatures are more likely to get knocked out and stay weak, while your strong creatures get stronger. (Note, however, that "gaining a level" doesn't mean that every level you get more HP or power. You gain HP or power only very gradually. When you reach level 50, which is the maximum, you generally gain another action, which can be extremely powerful.) You then have the opportunity to heal up and revive downed monsters, which requires the items you bought at the item shop, so make sure not to run out when you're far away from the shop, because then you might have to engage in some fights on the way back. Usually you'll want to rearrange your forces back to their starting lineup at this stage; irritatingly, you can't rearrange your creatures before battle, only after, so if you forget you'll have to start out with a suboptimal arrangement.
You can also switch out your creatures, if one of them just isn't working for you. This requires you to buy randomizers from an item shop, which (as the name suggests) replace your creature with another random creature (its level is kept, though). I don't understand the purpose of this mechanic -- it just forces you to keep clicking until you get the creature you want. (I suppose it gives you a chance to look at some creatures you wouldn't otherwise.)
As is so often the case with games of this nature, the beginning is entertainingly challenging -- not only do you want to win a battle, but you want to take as few casualties as possible, and your HP replenishing resources are scarce, so you have to fight carefully. Unfortunately, as time goes on and your creatures become more and more powerful, the battles become easier and easier, so that by the end of the game, its length definitely becomes somewhat of a drag. (Though I should say that Brute Wars is not really terrible as far as length goes -- there are certainly far worse offenders -- but it does drag towards the end.)
The sound is very basic, and the music also has a very generic RPG feel -- it's not great, but it's not terrible, and it does a good job of keeping itself unobtrusive so you're not heartily sick of it by the end of the game. The interface could use a little work -- I often found myself switching creatures and then hitting the "end turn" button instead of the "go back to attacking" button, which was awfully annoying. The game also does not autosave, so you have to remember to save frequently. This is kind of unusual in a Flash game, and it definitely is a minor irritant.
Overall, Beast Wars is an interesting little concept -- there's definitely a lot of fun to be had at the beginning of the game. However, the lack of depth means that you'll probably finish all of the excitement to be had before reaching the end of the game.