Question

During the Medieval period, when land was the only source of wealth and power, how would a nobleman increase his wealth

Answer 1

Many nobleman often owned mills, communal ovens, and other things of that sort. Also they could get money via taxes on market stalls, produce, and of course, fees paid by those who committed misdemeanors. A nobleman could increase his wealth via landownership by having slaves/workers farm the land and selling the literal fruits of their labor.