you don’t even need houserules…
page 10 states:
Up to one Infantry or Large Infantry unit
(including Individuals) may occupy each
section and is considered as “Garrisoning”.
in 3rd ed this means you could garrison with infantry, heavy infantry, ranged infantry, large infantry, or monstrous infantry. but not cavalry, chariots, monsters or war engines.
not sure where swarms fall on this. (at a guess alongside monstrous infantry.)
and as far as the rules go i’m not seeing any limitation on how many enemies can attack a wall. the only specification is that they have to attack the wall itself, not the unit garrisoning it, unless they meet special criteria.
Charges may be declared against a castle
wall section or gate from either side of the
castle wall, as if it were an ordinary unit.
A charging unit must make contact with a
wall section if they intend to attack a unit
the big thing is this one:
A Garrisoning unit may only be attacked
from the outside of the castle if the
attacking unit has Siege Equipment that
allows it to, or it is of Height 4 or more
though i agree that a 12" frontage makes it really hard to defend with only one unit even in that case. (though that does encourage you to use hordes and legions when available)
since you can multi-charge a unit normally though my suggestion would be to give the defenders improved defense. either use the defense of the wall itself (7+) to reflect the protection, or give the garrisoning unit the benefit of “big shield” and “ensnare”. which in many cases might work out to similar results.