The "Download Now" link directs you to the iTunes App Store, where you must continue the download process. You must have iTunes installed in order to open the link, and you must have an active iTunes account to download the application. This download may not be available in some countries.
From Michael Bergen:
Space Defender is an arcade style first person shooter. As enemies attack, use your weapons to defend yourself. Limited ammo, powerups, primary and secondary weapons, and joystick control make each difficulty level uniquely difficult. Space Defender implements Game Center, with several leaderboards and many challenging achievements. Try to reach all of the achievements and you will receive an upgraded weapon. Space Defender includes:- Arcade style controls- Joystick movement- Stunning retina graphics- Original music and effects (by David Bergen)- Leaderboards- Achievements- Permanent Lifetime Kill count