Facebook gives players the ability to earn crystals by completing advertiser offers. You can do so by clicking "Earn qrystals" under the game window. The system handling the interaction between "Knights: Clash of Heroes" and an advertiser (an app, a game, a survey website, etc.) is called TrialPay.
If any kind of an issue appears, e.g., you complete an offer but do not get the crystals within 24 hours, it means that there is a problem with either TrialPay or an advertiser. Please contact TrialPay for further assistance on the encountered issue: click "Earn qrystals" → "Help" → "See offer status" → "Report a problem".
You can also find more information in "Earn qrystals" → "Help" → "Frequently Asked Questions (FAQ)" or contact TrialPay directly using this form.