This is probably a dumb question but I just played my first game and was unsure how the Greatsword effect is triggered. By an enemy unit being assigned to attack a zone the Greatswords occupy or by playing aditional units into the zome from my hand? Just not 100% on this. Any clarification for me?

Greatswords is triggered by any unit entering the zone in which Greatswords are located. Zone here means your zone (i.e. your Kingdom, Quest or Battlefield, wherever Greatswords is located). Note that Greatswords also triggers itself when it is entering.

They are triggered by units entering a zone which means played from your hand, moved in from another zone, returned from the discard, etc. As Tako mentioned, they can trigger their own effect when they are played into a zone. I'm pretty sure that attacking units do not trigger them, though, since attacking units are not considered to be entering a zone. The reason for that is that after battle they are either sent to the discard or remain in whatever zone they came from, they are not put into the zone they attacked.

Greatswords are triggered by themselves entering their zone, but actually I am not clear how exactly that works. When Greatswords are played as an action during the Capital phase the "Play Card" action is put on the stack and assuming the opponent does not play any action the "Play Card" action resolves. During resolution, Greatswords is neither in play nor out of play. Once the action has resolved, Greatswords enter play (and their zone).

Now, I think put aside some specific actions, only cards in play can have their actions or effects triggered. So the Greatswords action can only be triggered once it has completed entering play, so the previous action has been fully resolved. At that point in time, why does Greatsword still find itself as triggering its own "Card Entering Zone" condition?