The blue sphere pertains to everything else, including energy, engines, and miscellaneous. With all three of these spheres to populate, you're given a wide range of abilities to bring to the table when facing your airborne enemies.

Speaking of which, even only playing the tutorial missions offers a lot of action. Once you're up to speed on controlling your craft, you're thrown into battle. Navigating the environment, you'll contend with the landscape, which can cause you to stall if you take enough damage -- and if you're too close to the ground when you do, you're boned.

However, Altitude doesn't follow the typical numbered lives model; when you crash, you respawn by hitting the 'F' key. So, the combination of respawning and gaining experience would seem to lend itself naturally to a stress-free gaming experience, but in reality it leads to a lot of vengeance-swearing and gritted teeth. Especially in later levels, when you have an actual objective to complete, and you keep getting shot down before you can finish the job. But even then, the game is simple enough -- and that's not a negative comment -- to keep things fun. There is enough to keep track of that aerial combat on a 2D plane is made compelling.

There are also gameplay challenges that Altitude offers. For example, you might be challenged to destroying three opponents in a row using the default plane. Completion of a challenge rewards you with experience. Additionally, you get a run down of interesting stats with each mission's completion. These are based on all players in the level, including bots, so you'll get things like "Kamikaze", which is who killed themselves the most often; "Best Kill Streak", which reflects who killed the most planes in a row, and "Longest Life", which refers to how long someone was able to go without being killed once.

On top of all that, Altitude can be played online. You just find the right server for your level and away you go, continuing to earn experience with each kill. If possible, these games are even more hectic than the standalone maps. If there aren't enough humans to fill the map, bots are added.

The demo is plenty of fun, but the full game offers more different types of planes, better upgrades, more maps, more challenges, more of pretty much everything. You can pick up the demo right here on Big Download for Mac, PC, and Linux.