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How did city walls affect the growth of medieval towns?

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Dense crowd of people walking through the brightly lit Shibuya shopping district in Tokyo, Japan, surrounded by colorful store signs and billboards in Japanese and English. — city Crowds of people in Tokyo's Shibuya shopping district.

City walls played a dual role in the development of medieval towns, acting as both a necessary protection and a physical constraint on growth.

Initially, walls were essential for security, defining the boundaries of a city once inhabitants determined it had reached its functional limit. However, these fortifications created a confined space that often led to significant challenges as populations increased. The resulting constriction caused overcrowding, poor sanitation, and a lack of light and air within the town walls.12

The relationship between walls and urban expansion was cyclical:
* Expansion: As towns grew and suburbs developed outside the existing defenses, many cities constructed new, larger concentric rings of walls to enclose the additional space.1
* Constraint: Despite these expansions, the physical limits imposed by walls meant that few medieval cities exceeded a mile in length.2
* Identity: The walls were central to the town's identity; the succession of these rings now serves as a historical record of urban growth.1

Ultimately, while walls were vital for protecting trade and food supplies, they forced medieval towns to develop in dense, often irregular patterns until the rise of later planning movements.12