Level 1 is the first Cathedral-style level in the original Diablo. The player(s) reaches it after entering the Cathedral through the main entrancy. Somewhere in Level 1 there is a stairway to Level 2.
It's also the first level where there is no quest related to this level.
Upon entering Level 1, the character says, "The sanctity of this place has been fouled".
While one might feel confident to stride through without fear, it may be better to fall back when in doubt, and player(s) should make sure to get free healing from Pepin when needed. Player(s) should also use doorways if necessary to make sure he/she do not get overwhelmed by Scavengers or Skeletons. Provided the player(s) make kills frequently enough, he/she/they will not be susceptible to swarms of Fallen Ones. The automap is the best tool the player(s) have for navigation.
The following monsters are always found in Level 1 (selection is not randomized):