Ben Mansell has never released a Doom level before, but his first effort took 300 hours to build, spread across an entire year. Originating as a doodle back in 2003, Foursite is an enormous structure, divided into four parts, each with its own theme and boss battle. Mansell reckons it took a friend three hours to complete on their first attempt and required some tinkering with "advanced processes" to fit the standard file format given its size. You'll need Doom II to try it out and there's a full dev playthrough below.
Before you take a look at the video, here's a description of how the level is constructed:
"The overall design for the map is a simple one: four wedge-shaped mini-maps, each with its own visual style and assortment of enemies. Each of these four 'quadrants' also ends with a kind of boss-room – some are environmental, some are enemy based, some are a combination of the two. Beyond each boss-room is a switch that opens the door to the next quadrant. Once all four quadrants are complete, the final boss room is opened and can be accessed to complete the level."
I've only watched the first few minutes so as not to spoil the whole thing. A few minutes is sufficient to get an impression of the scale and there's a decent amount of praise over on reddit for the actual design. Size isn't everything, y'see, and if this were a slog through one killing floor after another, it'd wear out its welcome long before it was done. Talking to Eurogamer, Mansell explained the kind of level he wanted to build:
"I always enjoyed the more environmental elements of the original games: the complex levels, the non-linear progress, the environmental storytelling. So I tried to build a map that didn't just rely on shooting enemies for the enjoyment.
"I think the ideas I'm most proud of are the environmental ones: for example at the end of the first quarter there's a room where you have to run across platforms while Mancubi shoot at you. That's the kind of environmental puzzle you don't get in that many modern Doom maps."
Foursite was constructed using GZDoom Builder and you can download it over on ModDB. You'll probably want to play it within an updated Doom engine like GZDoom or Zandronum. For more Doom maps, check out the Cacowards 2016 nominations thread over on Doomworld, where the best of the year are celebrated.