Came to think about, not as much what I prefere to hold a bridge with (it more or less been proposed above already), but how a situation may be handled when you do not have a desirable defence force and yet have to win, hehe.
At occasion, by no choice as far as I can remember, I've stood with few infantry, few to no missle units and a couple of cavalry units against a superior host on the other side of the river. Not the dream situation, we can agree
Taking enemy missile capacity into account, by moving up my own infantry [green dots on image] first in last possible moment, I positioned the units on either side of the bridge head, but leaving a gap straight ahead open. The aim is to catch the enemy advancing in melee with the infantry, so they get stuck at the bridge head rather than push through the open gap, where they flank the infantry and/or attack the cavalry. To that purpose, I more in detail position the infantry slightly angled, as a cone.
Already I have position the cavalry [orange points] in a line facing the bridge, and the gap between my infantry. As the enemy enter our side and stops, fighting the infantry, I initiated a micro-managing assembly line-tactic, where each cavalry unit in turn make a charge into the enemy units, are pulled out and repositioned so it may soon charge again, while next cavalry unit is sent into the fray in the same manner.
In time the grinding may break the enemy morale, and you win a victory. Well, if you're lucky hehe