www.themightyquest.com
Just got this game and its pretty cool. Currently in Open Beta. It is free to play but there is a cash shop. You dont need to buy anything to play and have fun.
Basically everyone builds their own castle. You design it and put monsters/traps inside to create your own instance basically. Then people try to beat your castle and you try to beat other peoples. The castles are the instances you run to get loot/exp. Also if anyone beats your castle then you can get revenge by attacking theirs.