5 - Keep: The fortifications in the fief. The monies spent are used building and maintaining walls, towers and other defensive structures. Spending money on the keep makes it easier to maintain loyalty and to defend the fief against external enemies. If you don't spend money on fortifications, your keep level will decline.

The maximum you can spend per capita is 13 ducats per capita per season. If you spend the maximum, the keep level slowly increases. Over a decade or so, this can amount to quite a bit and make your fief more difficult to conquer. Current Keep Level is in parenthesis. The higher the better.