Used to love coming home from school to find Dad had already started the PS2 and was setting up a 2 player game on Defender. We still never beat a particular map(s?) that I'm convinced requires 2 players to pull off after hours of attempts made solo. Went through so many disks as a kid that we probably spent $60 worth on how many I bought.. Sadly the only way to play now is emulation, or a on PS2/Xbox due to Midway games failure causing licensing to be lost. The game never made PC, and was never even brought to the newer generations of consoles.
For the diehards out there and GOG as well, please see about getting the Xbox version of the game. It had more maps to play, and the hardware allowed a special map where the stargates actually worked to transport your ship to another area of the map!
I'll note that the name used states "Defender-1983" , but the image is from the console modern remake. So I'm a bit confused on which one your referring to in this game page. Both have license issues as far as I'm aware, though I'm not sure if they're under 1 license (Since their the same IP) or 2 licenses. It would be cool if both could be recovered.