This; the men at arms that formed the garrison of royal castles and the core elements of household troops of the richer nobility would be professionals and pretty much mid level by the standards of the time.
0-level are essentially the peasant levies.
Now what consitutes mid level is a whole other kettle of fish.
Well, if the Lord is a 9th level Fighter - the minimum you get with the 1e DMG fortress generator - it seems reasonable to stat his 18-25 core household retainers/personal guard around half that - 4th, or say 2nd-6th. If the Castellan of the Keep on the Borderlands is 6th, then it seems reasonable to make his top dozen or so men 3rd, or ca 2nd-4th.
BECMI D&D tended more to this approach (see eg the Jarl's Personal Guard in the Northern Reaches Gazetteer), perhaps one reason why I found Mystara a more playable setting than Greyhawk.