From Sonic Retro
Seaside Hill is the first level of Sonic Heroes. Traditional beach and chequered hill fare, the stage includes palm trees and sandy shores. What distinguishes it from its predecessors are the sunken alabaster ruins looping through the surf just off the coast; there is even a massive whale-shaped island off in the distance.
The split-story nature of Sonic Heroes means each team has a different motive for fighting their way through this oceanic paradise. Team Sonic is out to stop "Dr. Eggman" from taking over the world after receiving his message during the intro. Team Dark, meanwhile, is also after Eggman, but each for a different reason even within the Team: Shadow to learn about his past, Rouge to get ahold of his treasure, and Omega to take revenge on Eggman for sealing him in the chamber with Shadow.
On a less malicious note, Team Rose blasts through simply because they're following Sonic; Amy for the obvious reason, with Big and Cream & Cheese tagging along due to a newspaper article depicting Sonic running off with Froggy and Cheese's twin, Chocola. Finally, Team Chaotix is being tested by the mysterious voice from the walkie-talkie, tasked to collect hermit crabs to demonstrate their detective prowess.
The normal / extra missions are: