Territories are workloads, not areas
The most common error in territory design is treating the map as the object. A city is divided into neat zones, each with a similar area, and the design looks equitable. Then one zone turns out to contain three industrial clusters and a wholesale market while another contains largely residential streets, and the two salespeople have entirely different jobs while carrying the same target.
The unit of design should be the working week. How many meaningful contacts can one person make, given travel, in the way your business sells? Build territories that fill that week with worthwhile work, and the map will look untidy while the workloads will be sound.