I’m telling the story from a pub in the bailey opposite the ruins of Oswestry Castle. There in 1165, Henry II, the Angevin (Anglo-Norman) king of England, gathered troops from the Marcher Lords to advance on an alliance of Welsh princes for control of the Marches.

The previous Sunday, 861 years later, we went the same way as Henry’s army to Chirk Castle, Castell Y Waun a’r Ardd, to see a re-enactment of the Battle of Crogen. The present castle wasn’t built back then and the battle at Crogen was in the Ceiriog Valley below. Henry had sent woodsmen to raze woodland in the valley to remove the Welsh advantage, but he left one oak, Quercus petraea, at which Owain Gwynedd gathered his army.

That tree became the Pontfadog oak – a giant, the oldest British oak, and survived as the National Tree of Wales, much loved and storied until it blew down in a storm in 2013 at an estimated 1,600 years old. I have a small fragment of it given to me by the longtime Guardian environment editor John Vidal.

The battle re-enactment, with crashing swords and shields in the colours of Marcher lords and Welsh princes, fine horses and boinging arrows, was fun to watch with a running commentary giving a sense of what was happening and who was doing what.

Histories vary about whether this was a great battle, isolated harassments or no fighting at all, but the outcome was the same: the Welsh won. The history of the Marches, like the traumatised, ghost-ridden ground itself, has been a contested space for centuries.

It was fascinating to talk to people dedicated to making the past come to life for modern audiences. Over the castle garden wall, the past and present were weaving together in micro-dramas of moment. Two clouded yellow butterflies, Colias croceus, which had migrated here from north Africa or southern Europe, were flickering over herbaceous borders and lawns. Given the massive decline in insect populations, that pastoral scene had a spirit of resistance.

And as if to demonstrate a mystical kind of heraldry and valour of a far more ancient territorial warrior, a small copper, Lycaena phlaeas, flourished his wings.