Level S-13 is the last Space Egg Level in Red Planet. To unlock this level, you must 3-starring from level 5-21 to 5-30.
The fortresses can be hard to reach for some.
Send the first Space Egg towards the steam geyser, and land next to the Corporal Pig sitting on the pile of Small Asteroids. Flying debris should detonate both TNTs and clear most of the level. Use the second Space Egg to destroy the remaning pigs on the left planet. If necessary, quickly launch the last Egg for extra points through the southern hemisphere. It should land on the right side of the main stone structure.