The Grendel Invaders metaroom in Creatures 3 is the layout for a Space Invaders game.
Open up the CAOS Command Line with SHIFT+CTRL+C and type in:
meta 6 1 1 1
Installing Vampess' Fixed Star upgrade allows you to access the game and play it without typing the CAOS command, provided you can find the location of the easter egg to click on it.
Hint: If you right-click the blinking star with agent help activated, you'll see a cute poem.