In this paper, we examine the relationship between built-up area residential density and organized violence. Security for most of human history has been one of the driving factors for city development and growth; this remains true today. What is new, is that the prevalence of organized violence is associated with increased population density in adjacent built-up areas. Throughout history, those fleeing organized violence have sought refuge in areas that provided “pockets of safety” or a measure of “relative security.” We first observed this phenomenon in Colombia when we found that after decades of organized violence between guerilla groups and the Colombian national government, its cities were denser than cities in neighboring countries. Drawing on data from the Uppsala Conflict Data Program, we found that in a representative sample of 200 global cities, cities with frequent organized violence were also associated with denser residential built-up areas as well. We conclude that organized violence creates an invisible wall that contains the outward expansion of cities adjacent to organized conflict.