No Population Decrease In Attacked City

Playing Civ 2 with 242 patch & XP.
Enemy cities with no walls do not loose population when attacked so I can't get rid of them. I have not played for long time but I remember easily destroying small enemy cities in my territory, but now can't.

The city may not have walls, but do they belong to the civilization that built the Great Wall?
You have also the solution of putting troops in all adjacent squares will prevent the city from getting the corresponding resources, eventually starving it to death.
Finally, if you happen to win the city, build only colons: each one will remove one population.