International visitors to Forbidden City in December

Hello,
Can you advise if it is MANDATORY to pre-purchase tickets to the Forbidden City if you are an overseas visitor?
I have read it is recommended to avoid crowds during peak season (and if you are a Chinese resident), however I will be there in early December and have read it is not likely to be very busy and that there is a separate "visitor" line. If I do pre-purchase tickets through TravelChinaGuide, do I have to line up to get a separate ticket or is the ticket I receive online get me straight in?