It is the dawn of a new year. The old king has just died childless, and the new succession must be decided before the end of this year. You are a local baron or countess with as good a claim as anyone else, so now is the time to grab the throne. Your peers understand only force, and your subjects understand only gold. Gather your men, pay your knights, and assert your claim. Everyone will recognize the strongest and richest as the rightful monarch.
In Melee you want to be the first person to capture an opposing castle (at which point the game ends immediately) or to have the most land and gold at the end of the year. Money is tight, and gold is used in the game both to build new units and to attack. The more gold you have and spend on your troops, the more motivated they will be and the higher your probability of success.