A City Space is a space on the Eldritch Horror game board marked by a blue "building" symbol.
The Investigator may undertake a number of different actions during his turn while on a City Space that he may not be able to undertake on other spaces. The Acquire Assets or Prepare for Travel actions may only be undertaken when in cities.
Certain Assets, like the Urban Guide, provide boosts only on City Spaces. Darrell Simmons gets a bonus die when resolving encounters in City spaces.
Some cities may be devastated by the result of a Disaster. If that happens, they no longer count as City spaces, and Actions and abilities that must trigger in cities do not trigger on a devastated space.