Catapults, Sentry, Immortals-Red Dargon - they are defending your castle. You can buy and upgrade them only for sapphires.

After you are beeing raided they will be free revived in 3 hours. You can use sculls to decrease revival timer 50%.

But frankly speaking the folowing are more usefull:

• Decreasing darkening cost (-50% cost);
• Decreasing food consumption (max -95%=85% (spring) + 10% (obelisk))
• 3 additional maximum raid attemts;
• Increase of combat experience.

These 4 are the most usefull.

You can also spend skulls on increasing training speed on certain unit.

It's up to you =)

Decreased Darkening is ONLY useful IF the league you belong to owns a beacon, if it does not then its is completely pointless, in my opinion Guard revival is more important than decreased darkening .