Going Down for Doom II is modification (technically speaking, a "megawad") composed of 32 new levels. It was created by Cyriak Harris (mouldy) and released in 2014.
The mod, which won several awards, features an original soundtrack and small, hard maps filled with enemies. Each one of the 32 levels has its own style and graphics, which is really high-quality.
When you start, shoot one of the guards at the end of the road, then walk around the block and look for a building with barricades in front. Walk inside and turn right to the bright red push button. Press space, and the elevator will take you where the action is.