In the League of Legends wiki it is clearly stated "Strangely, the nation [Piltover] is situated over a vast bubbly green swamp."

Nevertheless, the artwork by Matt Gaser made the city look like a coastal city built on grassy terrain,

http://images4.wikia.nocookie.net/__...x-Piltover.jpg

Actually, just looking at the image, and where it is on the map, it IS coastal. Florida-Everglades-type coastal. Due to their incessant work, the water would be mostly clean (though still a bit murky) and if you notice: There's a greenish haze all across. The city probably started off being built atop that hill, above all the muck, and slowly expanded. Granted, the water does look too clean for "bubbly green swamp", however, maybe it's just a bubbly swamp with greenish gas (their power source) leaving it have an overall greenish hue.

Also, check out those two houses in the bottom right, they look like they're right off of a greenish swamp, rather than next to grassland.

Judging from Caitlyn's background, the place is a little too moist to be a grassland.

http://leagueoflegends.wikia.com/wik...er/SkinsTrivia

Though I agree it's not moist enough to be considered a swamp. Perhaps it would be better to call it a big "marsh", which is between a swamp and a grassland.