Start a crusade and defend your land against your enemies, who will not leave you alone! They are attacking your land, you'll have to fight back at all costs and make them understand that they can not attack you without paying the price. Place soldiers, pikemen or archers on squares so they attack the invaders. Do not let them reach your castle, otherwise you will fail and you will have to start over. Use your money to improve your units, or to place new ones on the battlefield. Good luck!