The first secret level. I've always loved playing this level for some reason, even though it's a bit of a difficulty spike in the sense that all enemies are fair game, not just ones that have been in the game up to this point.
The hardest part of this is easily getting out of the room with the yellow key. One of the spawns that triggers is those obnoxious missile launcher platform robots (those snarling skinny green things). On Insane difficulty, just one of those can take you down from full shield in one burst of fire if not killed quickly. So the important part is... well, killing them quickly.
Another thing worth noting is that destroying the reactor is actually possible to do early by shooting through the grate in the secret room behind the yellow key, but I choose to opt for clearing the entire level of enemies instead.
Currently, SpootyBiscuit has 40,820 views for Descent across 35 videos. The game makes up 5 hours of published video on his channel, or 2.40% of the total watchable video for Descent on SpootyBiscuit's YouTube channel.