Port Town Aero Dive is a racetrack from F-Zero GX. It was also a stage in Brawl, and was brought back in Smash 4 instead of a new F-Zero stage. Granted, there was a new F-Zero stage in Smash 4, just not on Wii U. But on to Port Town, this town was where Captain Falcon was born according to the manual for the first game, which is apparently canon to Smash because that's where the Falcon Flyer came from. Now you know. And knowing is half the battle. G-I-JOOOOOOOOOE. But anyways, like Melee's Mute City before it and the Mario Kart stages succeding it, Port Town takes place on a platform that follows the racetrack and stops at various points. The stage also has F-Zero racers coming in at high speeds. If you are playing All-Star mode, they will always come, they will always hit you, and you will always die from it. Every time.
Also, this stage has a giant ROB near the track, just like in GX. Neat stuff.