Press the green flag and play! (Also, when Emmaline says “things that will help you,” she means catch the different gems. And when you want to go to the final fight, press the ‘f’ key and if you want to go back to the map of the castles, press the ‘b’ key.
Also when you click on the castles, you can go to that realm/castle/kingdom/whatever-you-call-it. And, if you want to beat the “deadly monster, ; )” you have to get at least, 15 gems.