My questions regard the whole 'Free to play' thing and payments.

So I love games like this and I have played (and payed for other games too). I was planning on, when I get payed this summer to pay for an upgrade for this. But then I look things over for it and I realize that after a month they 'force' you to pay? Is that true? Is this more of a free trial thing than actual free to play?

Im not bashing it or anything I just want to understand what it is...I love this game but I don't have the money to pay whenever they want me to... So if you would be kind enough to explain that would be great thanks guys!