That is correct, there is a turn limit. Just like there is one in Civilization. The game doesn't stop at that time, but your score will be frozen. This is to make highscores more meaningfull, or somthing like that.
Funny, I always thought turnlimits were a rubbish idea!
Whenever I play civilization, I always turn them off when possible, they're terrible!
Sincerely
~ Kongdej