I'm not sure it's possible to make secure. To render the positions of the enemies, the browser receives 4 coords. To submit the capcha, the browser submits 4 coords – the same ones it received. Perhaps you could track the variance between the exact position and the position the user selected, as well as timing. But would it be enough?

I think it's safe enough for many cases. Amazing concept to have little classic games as Captcha.