There are three Castle fields on this page. The first, is a simple type the random number in the box. The second, type the current year, using the date function to obtain the current year. The third is a honeypot, and is hidden using CSS. You will also notice that you only have five minutes to submit the form.
A program could be configured to treat a time out the same as a wrong answer (the case for the first field), or differently (the second field and third fields).