Hell is similar to the Caves in structure, with large open room. It does however feature corridors and distinct rooms, though it offers no narrow chokes for the player to use to their advantage. Naturally it features some of the toughest and most iconic enemies of the game.
Though it is called the Hell in game, it is doubtful whether it is actually hell the player is in. In Diablo II, it requires the collective power of the Three in order to open a gate for Diablo to step through, which seems to contract the rather easy way through which he managed to get there in Diablo I. It also looks radically different, with lakes of blood behind the walls, as opossed to the stone and lava seen in Act IV in Diablo II. It is possible the Hell in Diablo I is another part, or world, or the Burning Hells.
Upon entering Level 13, the character mentions, "I must be getting close."
Warlord of Blood: The Steel Tome will tell the player of a gruesome commander in the armies of Hell who has slain countless. Bartuc, as was his original name, is triple immune even on normal, but defeating him will yield access to several magic items and sometimes even uniques.
Lachdanan: Slowly being consumed by the curse that Leoric placed upon him, Lachdanan seeks a way to save his soul from the curse. If the player helps him, he will part with his Veil of Steel as a reward.
Archbishop Lazarus: Cain will task the player with killing Lazarus and saving Prince Albrecht. Found in a separate area on this level, he is guarded by several powerful servants. Though it will at this point be impossible to save the prince, completing the quest will unlock the portal down to level 16.