MB4 264a: Measuring City Borders and Techum (398:2-4)

If a city is circular, triangular, or another non-rectangular shape, the 2000 amot are calculated by squaring off the city, adding the uninhabited areas within the square. The city is squared off by measuring 2000 amot due north, south, east, and west.

A city shaped like an "L" or like an arch we view the empty space in the squared-off area as if it was part of the city up to 4000 amot