Glyndwr is, of course, a local hero. I drive through his lands at Glyndyfrdwy once a week on my way to Llangollen and G and I once visited the site of one of his most famous battles at Hyddgen.
Anyway, during his long campaign against the English some of his supporters managed to take Conwy Castle.
This site says the Tudor brothers "snatched Conwy Castle by a trick". Wikipedia says that Gwilym and Rhys ap Tudor easily took Conwy Castle while the garrison was at church.
The S4C programme, however, claimed to have more details. To whit... By the time Glyndwr was rebelling against the English, Conwy Castle was over a century old. The roofs were leaking and the supporting timbers were rotting. it was as difficult to get builders then as it is now.
So when they heard someone pounding on the door they naturally demanded to know who it was. And on hearing that it was the builders come to mend the roof, they opened the gates...
Probably not a ruse that would work in a high fantasy, though it's worthy of a Pratchett protag. *g*