nope, leave them empty and have the armies in adjacent provinces.

after the horde appears, then move units in.

you have max of 960 men so use your elite troops, preferably heavy defensive inf like varangs in your case.

let the fortress wear them down.

also guard Kiev.

have some heavy inf to serve as bridge plug and have loads and loads of archers to shoot down anything that moves.

in bridge and castle defense, some catapults are always nice.