When defending bridges I rely ehavily on missile units. I usually put a good spear unit at my end, especially if the AI is attacking with heavy cavalry. I use two or three javelin units right behind the spear unit - I take them off the skirmish mod (otherwise they retreat outside of their range), and wait for the enemy to approach. Usually the AI attacks with its best unit first, but if it sends a relatively weaker unit I preserve the javelines for later and only use bows or crossbows (arbalests if I have them). As long as the spear units holds the initial impact for even less than 30 seconds, just enough to allow my javelinmen to do their damage, this tactic has spectacular results. The best I can remember was using three units of lowly Slav javelinmen to kill two units of Variangian guards with armor upgrade and plenty of valor - nothing nicer than to see how the elite of the Eastern Roman Empire disappears in a matter of seconds, and it is a rare sight indeed to see a few Variangian guardsmen from a 7 valor unit on expert to run away, greatly diminishing the morale of the rest of the Emperor's army. The only weakeness of all types of javelinmen except armored Almughavars is that they are usually not armored and get many casualties if within range of the enemy missile units.