Base of Operations is a short adventure intended for four 5th-level characters. DMs can easily modify the adventure to suit higher- or lower-level adventurers, or larger or smaller parties of adventurers. Simply adding a few monsters to every encounter area makes the adventure more challenging for larger parties, and adding levels to any of the humanoids can make them more of a threat to high-level groups. For low-level adventurers, make the relationship between the two factions within Brightstone Keep more strained, and take away a few monsters from each group. You can remove levels from some of the humanoids in the adventure to make it a lower-level challenge, but it is important that the orc cleric (described in encounter area 8) still have the ability to animate the dead. Still, he can have fewer minions around him when encountered, and that makes him less of a challenge for a lowlevel party.