Deciding how enemies spawn in each lane requires careful planning on the part of the level designer. Each of the three lanes — ground, air, and the optional, sub-objective blocked lanes — are balanced differently, and each designer has different balance goals depending on the map being created. Ultimately, though, the goal of all level designers is to provide a fun and challenging experience for the player.
We want to avoid the need for players to build the same defensive structures in every lane, as we feel it robs them of any sense of choice or agency. Until recently, we had a limited selection of core enemies to use — just the standard Orcs, Goblins, Kobolds, and other monsters from DD1. To promote as much spawn diversity as possible, every ground enemy was placed in every ground lane, creating situations where you knew you were going to have a certain mix of all enemy types in each lane.