There was a massive Roman fort at Llanfor, the site of the 1997 and 2009 National Eisteddfod.

We think it's an early fort of mostly wooden structures, dating back to the first Roman invasion of north Wales. It's only been detected from aerial photographs and geophysical surveys.

But the results turned out to be the best I've ever seen. Because of the soil conditions, the outline of every building within the fort, and rooms within those buildings, can be seen.

Geophysical surveys detect changes in the soil due to its magnetic properties. If you take a field which hasn't been disturbed since it was created in the glacial age - or by rivers, in this case - the soil will be even right across.

But if a ditch has been dug, disturbing the top soil, or if something has been burned on the land, then the changes in the magnetic oxides will be picked up by our equipment.

We take a reading every 50cm across the field and then create a picture from all the readings. You can just see the outline of the fort if you know where to look.

This fort was picked up by accident by an archaeologist flying back from surveying another Roman fort, Caer Gai, at the other end of Bala Lake. This was during the drought of 1976 when lots of hidden sites like this came into view.

We returned before the Eisteddfod came in 1997 to ensure nothing would be disturbed. As huge ground anchors need to be driven into the soil to support the pavilion, it isn't built over the fort itself - that's closer to where the car park will be. The cars won't do much damage to the land.

There are also some Bronze Age burial sites nearby and something which might be a Medieval enclosure. A young archaeologist might be investigating this while the National Eisteddfod is on.

The site of the eisteddfod in Bala certainly has a very unique history. Dave Hopewell