Altitude puts you in charge of a small aircraft with a deceptively simple control set. Left and right arrows control ascent and descent (the game is a 2D shooter). You can slow down or speed up by way of the throttle controls -- down arrow to reduce speed, and up arrow to increase speed. You also have access to an afterburner by pressing the up arrow again while at full throttle, but this doesn't seem to increase speed appreciably.

Instead, the afterburner helps you recover from a stall, which can happen when going vertical for too long, which as avionics fans know puts a lot of strain on the engines. Once you stall, you begin to plummet to your death. The only way out is to turn your plane nose down, into the dive, and hit the afterburner. This powers you out of the stall and brings you back.

The afterburner is limited by your plane's supply of energy, which also powers your weapons. Always displayed over your plane are three meters, from top to bottom: speed, health, and energy. For the most part, gameplay is hectic enough without your needing to worry about keeping an eye on any of these. Speed is something you'll pretty much keep at full throttle at all times; you'll be constantly looking for health at all times; and energy replenishes fast enough that you'll quickly develop the necessary pauses to allow it to regenerate.

The 'F' key fires your primary weapon. Depending on which weapon you're using, this can use up energy. The 'D' key fires your secondary weapon. This is typically a one-shot deal, but worth using. The 'S' key deploys your special weapon. This is an item you'll pick up while moving around the playing field.

What's interesting about Altitude is in the number of options you get to play with. First, as you play and rack up enemy kills, you'll gain experience. Though it isn't in your face or terribly explicit, once you've leveled, you gain access to new abilities, new planes, new game modes, etc.; your plane itself doesn't gain attributes.

Second, before each mission, you can choose which plane to take into battle, and furthermore, how to kit it out. There are three spheres of influence in which to improve your plane, coded by color. The red sphere pertains to your plane's firepower, and upgrading it also upgrades your secondary weapon.

The green sphere pertains to your plane's hull, and affects elements like durability, flexibility of wings, and repair capability.