Hay Day is one of the most challenging games I have ever designed for.  It has such a unique look and feel that if you have to be incredibly careful.  That doesn't mean you can't be creative with the property, but you have to work within very strict limits.  The challenge each new project or addition brings is always exciting.  Not to mention that both my parents play Hay Day....a lot....and they are always quick to tell me what they think.