Shield Wall Fighting - how would you model it?

[Yorunkun]

Imagine a typical Dark Ages battle between two groups of a few dozen sword- and spearmen each. The defenders line up in a shield wall, the attackers try to break it. How would you model this situation in GURPS, using the advanced combat system (and martial arts, if you like), aiming for realism?

Specifically, ...

- what (dis)advantages does being in a shield wall bestow?
- what actions would the defenders typically take?
- how would you have a group of attackers try to break the shield wall?
- how would you model massed slams (a wedge of attackers shoving into a group of defenders) ?

Also, would you recommend running a fight like this, considering the high number of combatants?

Thanks!