Overhand spear cavalry

[Cullhwch]

In that role, then, do they (as in hippeis-equites type cavalry) offer any sort of advantages over the sword-armed skirmisher cavalry from the Hippakontistai to the Celto-Germans? I know that they have a higher lethality than the kopis cavalry, but the rare longsword horsemen will have an edge in both throwing javelins into the backs of the skirmishers and in killing them in the ensuing melee. I'm sort of at a loss as to how to use hippeis effectively. Are they only useful if nothing else is available?