Quote Originally Posted by Ars Moriendi
Yes, they can work beautifully, but only in a very limited set of circumstances :

- What about fighting against horse archers ? An all inf army can't deal with that...
It absolutely can. Since half my army will be high quality foot archers or pavise crossbowmen, I can cut his horse archers to ribbons while my much hardier troops easily weather his weaker arrows.

- What about an heavy cav enemy ? You'll just get surrounded and mass charged to hell from all directions...
I've set myself up with all infantry armies versus full on crusader armies (Heavy Cavalry dominated armies with light infantry support) and turned them back easily. Pikes can stop Heavy Cav easily, and even Armored Sergeants or some form of Heavy Infantry can hold them and inflict grievous casualties while your crossbows volley and break their morale.

It's also completely possible to deploy in full square, or to lure the enemy into charging the wrong point and envelop and overwhelm them with superior numbers (150 infantrymen per unit vs. 80 men per cavalry unit).

Heavy cavalry is overrated.

- What about fighting against massed archers on a hilltop ? Your all-inf. army will get shot to pieces on the way...
This is trickier, but it can be done. The fact that they are taking a stationary and defensive stance allows you to split your forces and envelop the enemy. The AI seems to have no concept of the "square" formation.

There are other strategies to deal with this situation as well.

- What about achieving the ideal victory ? That would be making the enemy rout as quickly as possible with minimum engagement, thus limiting your losses and maximizing theirs in the rout. You can't get them to mass rout quickly with an all inf army (that would require cavalry rear charging the engaged units) and if you can't catch the routers...
I think you underestimate the ability of massed crossbows to create routs.