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I checked both the blackout basement and absurdly spacious sewer page, I think, and didn't find anything that stood out.
Some aspects remind me a bit of Iji, but while there was a dark basement in there, there was no sewer.
"And as long as a sack of shit is not a good thing to be, chivalry will never die."
Any chance it was a ported copy of the NES game Fester's Quest? It involved an alien invasion, sewers and I'm pretty sure basements. Plus it was an 8-bit game, so it would look pixel art style.
It was not Fester's Quest. It had a side view and was a platformer. I believe you could climb the buildings and reach the roofs.
I'm thinking Bio Menace, Duke Nukem, or Duke Nukem II.
The game involved an alien invasion in a city that you fight for. I think that you had to go into the sewer system a couple of times. It had a pixel art style and you played it on the computer. I believe the guy had some weapons to fight the aliens with.
Edited by Dijini