The Areas in Diablo I are all accessed from the town of Tristram. The original four are all accessible through the Horadrim Cathedral, while the two areas added in the Hellfire expansion are separately placed. Each dungeon consists of four levels with new and more difficult enemies located further down.
In the original game, there are a total of sixteen levels divided between four differently themed dungeons: the Cathedral, Catacombs, Caves and Hell. In order to reach the Catacombs, the player must first pass through the Cathedral. Spread across all these levels are a total of 16 quests (not spread evenly) and a multitude of unique monsters and bosses such as the iconic Butcher. All of these quests and bosses are optional (up until the Archbishop Lazarus quest) and in order to progress the player simply has to find the stairs to the next level.
The two levels introduced in Hellfire are the Nest (informally known as the Hive) and the Crypt. The former is of similar difficulty to the Caves, the latter to Hell. Unlike the original dungeons however, these do not feature any additional quests and only one new unique, Na-Krul, at the bottom of the Crypt. They do have a variety of new monsters though.
- Tristram is the single town in Diablo I where all the shopkeepers and remaining townspeople live.
- The Cathedral features perhaps the most iconic levels and music of the game. It is home to both The Butcher and The Skeleton King.
- The Catacombs are darker and more cramped than the Cathedral, with tight passageways and few doors.
- The Caves lie below the actual cathedral building and are, as the name implies, large open caverns with lava and wooden palisades strewn about.
- Hell, the final area, features large open rooms and wide corridors with both aggressive and dangerous monsters. At the bottom are both Archbishop Lazarus and Diablo.
- The Nest is of similar difficulty to the Caves and looks like a slimy, organic dungeon.
- The Crypt is of similar difficulty of Hell, but looks more like the Catacombs.