As long as we’ve had cities, we’ve built ways to keep people out of them. In early times, guard towers and pointed palisades kept wanderers away and enemies at bay. Today, urban designers employ similarly defensive measures throughout the public realm, in subtler and more nuanced ways.

Increasingly, elements of “hostile architecture” are found in places meant to be accessible and inclusive, such as public parks, city streets, and hospitals. While the intention of these features is often rooted in durability and safety, the effects can be more wide-reaching, says Cara Chellew, a research administrator at York University in Toronto and an advocate for public spaces.  “It’s sneaky, because a lot of features seem benign and innocent, but are actually used for other reasons,” she says.