A straightforward concept which makes for a very intense and rewarding challenge!
Interestingly, I found a mouse and a trackpad had their own advantages: with my mouse I could aim easily but found it harder to flick, whereas massive flicks with a trackpad were easy but I sometimes missed the enemies I was trying to pick up. I wonder what it'd be like on a touchscreen...
It took me quite a few tries to finish the 60 seconds without my castle taking any damage, so I'd say the difficulty is just right too.
Nice work!