The ROMs for the game Joust in this release contain the "green" ROM image. The green ROM fixes a well known bug that allowed you to endlessly kill Pterodactyls on certain levels, thus achieving incredible scores.
In the links section for this game you can find information and a patch program to undo the "green" fix and convert your ROM image back to "yellow" and take advantage of the bug!
Jeff Vavasour optimized as much as humanly possible with this emulator; the specs say that a 486 is the bare minimum, but you can play the Robotron emulator on a 386/33 and have very minimal slowdown.
Defender II is better known as Stargate- after the arcade release, Williams determined that the game's name infringed on an existing trademark (apparently for a board game), and the name was changed.
Despite Williams' assertions, though, the game's name was not changed immediately after the arcade release. The disc reveals that Williams programmers had to go to Atari to reprogram the title screen for the already-finished 2600 version... the problem is, the game was in release *as Stargate* for the 2600 later in the year- the Defender II variant didn't show up in stores until 1988. Unless the 2600 game was programmed and released at the same time as the arcade game, Williams' timeline is faulty.
(After the release of the movie Stargate a few years ago, several related home video and board games were released, without trademark conflict.)