The Lancea is a 9-foot ash pole mounted with a 14-inch spearhead. The spearhead had a wavy pattern, which reduced friction and allowed it to penetrate more deeply into the body, and to cause more trauma when it hit.
The Celtic lancea was used both as a thrusting weapon and as a throwing spear. Its great size and serrated head made it highly effective in battle, capable of piercing light armor that the Celts commonly saw on the battlefield. In the show, lancea went through a pig carcass when it was thrown as a javelin, and came out on the other side of the pig, far enough to hit another target. However it was noted that once thrown, the weapon was lost as it would be very difficult to recover it during a battle.
Celtic Chariots normally used 2 javelin throwers and a driver.
Celts were agile spearmen and used their spears' range and light weight to keep Roman Legionaries at bay. However Celts rarely used formation fighting like the Romans and would only have such advantages in 1 on 1 fights. Roman Scutums were also designed to parry spears, allowing the Roman to deflect the spear away and approach the Celt for a killing blow.
Some Celtic and other barbarian tribes did use pikes to adopt a phalanx formation similar to the Greeks. 
Celtiberians became famous for their use of javelins. Spain would continue to use javelins throughout Medieval and early Renaissance history; with the Almughavars were noticeable skirmishing units that specialized in using javelins.
Celtic Spear Fighting: