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.
Upon entering Level 1, the character mentions, "The sanctity of this place has been fouled".
It's also the first (obliviously) level where there is no quest related to the level.
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. Use doorways if necessary to make sure you do not get overwhelmed by Scavengers or Skeletons. Provided you make kills frequently enough, you will not be susceptible to swarms of Fallen Ones. The automap is the best tool you have for navigation.
The following monsters can be found in Level 1: