[Kungfoo]

From the manual:
Repel is how Dominions represents the effectiveness of long weapons, such as
halberds, pikes, and the like. If a unit attacks an enemy who has a longer weapon than the
attacker, the defender may actually repel the attack and prevent it from happening. This
occurs as follows.
Any defender that has a longer weapon than the one used by its attacker makes a repel
attack automatically upon being attacked. Attack and defense rolls are made. If the result is
a hit, the attacker must make a morale check or immediately abort his attack. If the attacker
passes the morale check, he may make his attack, but the defender’s attack then generates
damage and protection rolls. If damage is inflicted, the attacker takes one point of damage
and finishes his attack.
This simulates the defender successfully placing his longer weapon between himself and the
attacker. All of this occurs before the attacker’s strike is resolved. Note that units with low
morale are more likely to be repelled, and thus using long weapons against low-morale
troops is very effective. Also, units with claws and bites (weapon length zero) are easier to
repel.
A unit gets -2 to its Repel roll for each time it has been attacked that turn. So it’s easier to
repel the first attack than the second, and so on.

Translation:

Defender (guy trying to repel): Attack rating + DRN
vs
Attacker (guy trying to get past the repel): Defense rating + DRN

If the defender wins, then:

Defender: Morale + DRN
vs
Attacker: Morale + DRN

If the defender wins at this stage, the attack is over and you avoid damage. If the attacker wins the morale check, he then goes on to make a normal attack role vs the defender, but the defender gets to roll up to a 1-damage hit against the attacker's prot (woo!).

In other words, it isn't just the length of your weapon; it's how you use it. Confidence matters, too.