Sigil

This episode is a design lesson. In some ways I've found it to be better than Sigil II.

Quality over quantity. There are few monsters, but their placement is much more meaningful than in the original doom. The level design provides little room to move around and this makes Sigil much harder than the original doom. In the original doom it was much easier to let the monsters fight each other, not so much in Sigil. There are much fewer medkits, armor, potions and ammo. This forces the player to be more careful about wasting health and ammo.

Doom didn't support real 3D levels, yet the level design creates the illusion of room above rooms. I could argue that such limitations forced the level designers to be more creative. I'm quoting Mark Rosewater "Restrictions breed creativity". The depiction of hell in Sigil is much much more interesting than in the original doom. John Romero clearly learned a lot in all those years. Some levels are non-linear and you can complete the challenges out of order.

It's incredible that doom didn't had scripting built in. If you pay attention to how John Romero builds the sequence of events in each level you can almost be fooled and think that he must have used some source port with advanced scripting capabilities. He didn't. The cyberdemon takes a lot of damage to die, but there is a clever usage of telefrag in some levels. I knew this because every doom player knows that you won't kill a cyberdemon with a pistol or a shotgun. The fact that the cyberdemon is trapped inside a square area is a hint that you are not meant to fight him directly.

In the original doom most secrets were obvious by means of flickering lights, misaligned textures or some out of place decoration. In Sigil all secrets are really secrets. You have to look very carefully to notice some small crack or something that feels like a secret.

Modern design principles have been extensively adopted. For ex: you teach the player some trick that is reused many more times. You warn the player that there is a trap ahead by means of environment art for example. There is a clear learning curve and progressive difficulty from the first to the last level.

The soundtrack is awesome. Some of them seem to be similar to the soundtrack of Heretic.

This wad is 4MB but it's worth 10GB of modern games.