Use potions that equal the value of a group of monsters that you select to stop them from destroying the castle.

Played this one during the school holidays with the kiddo. The game adds different elements in each of the worlds to keep the game interesting while ramping the difficulty.

While the hardest levels aren't all that challenging for an adult, adding a self imposed time limit for each turn to try to do it as fast as you can certainly makes it a little more interesting.

The kid bailed a few worlds in. I suspect she'd worked out that I had tricked her into doing maths. And but came back near the end to see how I was going without her and she helped out a bit.