Pikemen are soldiers of the regular army of Men. 

Spear is one of the most simple yet powerful weapons in existence. It is very cheap and easy to produce and as opposed to sword has a chance to inflict double damage (which grows with experience). It also has a range advantage over sword in battle, so pikeman will strike first. It is invaluable for protecting castle walls making it very hard for enemy to climb them. On top of that spear negates trample skill of cavalry in the field. 

On the other hand there are drawbacks as well: spear doesn't parry, so pikemen are more vulnerable to slashing attacks, it is also very ineffective for castle assaults (imagine climbing a wall with a spear), but the worst dream of every rookie pikeman is to break a spear which is a common mistake. Chances to break spear grow if enemy has a quality shield. 

Spear has penetrative type of attack (as opposed to slashing by sword). Penetrative damage bypasses enemy armor while slashing destroys it first. The better the armor is the lower is the chance to penetrate it and the damage inflicted as well. 

Veteran pikemen do not break spears and have a higher change to inflict double damage which may be so powerful that can sometimes kill an enemy with a single blow.