Retroville is the home of Jimmy Neutron, Cindy Vortex, Sheen Estevez, and other characters from the Jimmy Neutron series.
Timmy first visited here to get an invention from the greatest laboratory in the universe, so that way he could present it as his project for the science fair and win first prize. Timmy's Auto-Poofer accidentally got mixed up with one of Jimmy's laser pens, so he remained trapped there until he contacted Jimmy, Cosmo and Wanda through Goddard.
After Jimmy stops Crocker from taking over Fairy World, Timmy has his godparents get both him and Jimmy back into their own universes. Timmy nearly gets disqualified from the fair since he had nothing for the fair, but then Goddard appears and Timmy quickly claims to Principal Waxelplax that it is his invention for the fair. Waxelplax found Goddard so extraordinary that she gives Timmy the first prize ribbon much to the anger of A.J. and his clones.