I googled "levy definition" and got this:
1. impose (a tax, fee, or fine).
"a new tax could be levied on industry to pay for cleaning up contaminated land"
synonyms: impose, charge, exact, raise, collect; rare: mulct
"the government's right to levy taxes"
enlist (someone) for military service.
"he sought to levy one man from each parish for service"
1. an act of levying a tax, fee, or fine.
"union members were hit with a 2 percent levy on all pay"
synonyms: tax, tariff, toll, excise, duty, imposition, impost; rare: mulct
"the levy on alcohol"
an act of enlisting troops.
Think of the levy as raising taxes to pay for defenses (rooks) and/or armies (knights).