Captcha Invaders
A CAPTCHA is a type of challenge–response test used in computing to determine whether the user is human in order to deter bot attacks and spam. The term was coined in 2003 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford. Now, solve this snippets of CAPTCHAs to protect a city in a dome. It's all on you!