Just finished Three's a Crowd. What a high quality WAD! This is what I was talking about. Each map tells its own story and a part of the overarching story. Every map is different and not a single slot is wasted on filler (in my opinion). My favourite level would have to be MAP29 Sheer Poison. Not because I think it's the funnest (I hate the constant damage) but because it clearly communicates through the gameplay itself and the map geometry (where you start in a fake warehouse but really you're in hell), that the IoS is done playing around and is really trying to kill you this time. Combined with the music it gives a sense of hopelessness. That death is inevitable and that your struggles are almost over; give up, you'll die anyway, soon you can sleep. This level communicates a sense of sadness due to the inhospitable environment coupled with the cruel deception at the start. The last level, MAP30, is the only IoS fight I've encountered which isn't a let down. The strength of the multi-map build-up of the IoS and the sacrifices of the Doomguys gives gravitas to this enemy (without relying on the spawner!).