On the Barbarian Cross map there is a bug in the editor (or game) that allows for neutral cities with no defenders.
Open the map and the 2nd island city down from the top (Birka) has the problem. If you look at the details of the city it has a limited 'set' production. At the same time it's marked as a 'powerful' city. The problem is that none of the limited set of armies is considered 'powerful'. As a result if you start a game on this map Birka will not spawn with a neutral unit defender. It will be completely defenseless. Once conquered I found Eagle production (one of the random allowed units).
KGB