There are three icons that show the Overseer which monsters to place in the Monster Start Zone during the Setup Phase: Specific Monster icons, Mercenary Monster icons, and Monster Metre icons.
The specific monster and number of pieces to be used is shown in one or more quadrants of the Room card. In the case of the Barracks card (left), the Overseer takes 4 Orcs (level 1), 1 Troll (level 2) and 1 Pit Viper (level 2) from the Monster Pool and places them in the Monster Start Zone on the playmat.
The Setup card for the current Catacomb Lord will indicate to the Overseer which level 1 monster should be used when a Mercenary Monster icon (?) appears in the top left quadrant of a Room card.
The Battle Room card has Monster Metre icons in any of the four quadrants. These icons give the Overseer the most freedom and strategic flexibility in choosing the monsters that will populate a room.
For each Monster Metre icon, the Overseer chooses a monster of the specified level from any family (colour) and decides how many pieces to place in the Monster Start Zone. It's usually best to deploy all available monsters pieces at once. However, the Overseer may choose to only deploy some monsters at first and summon the rest later in the Battle Phase.
Alternative: The Overseer may choose monsters for each Monster Metre icon randomly. Each Monster card has a corresponding Monster Metre icon on the back. If the Overseer must place a level 2 monster, they'll shuffle all the Monster cards with a level 2 Monster Metre on the back and reveal the top card. The Overseer then places any desired number of available monsters of that type in the Monster Start Zone.
Note: There may be conflict if, for example, the Swordmaster Ghoul is revealed after the Crypt Spider has already been placed on the playmat (they both share the same piece). In this case, discard the Monster card and draw another one. Repeat this process for any other Monster Metre icons on the Room card.