Yesterday’s filming appears to have taken place at Castle Ward, according to Nelson McCausland, MLA (a member of the Northern Ireland government) in this blog post from yesterday. He notes it was interesting to see how some of the buildings were turned into a “medieval castle”.
Interestingly, the main building of Castle Ward is a mid-18th century great house, and a unique one: it has both Gothic and Classical facades. However, on its grounds are two medieval-era structres. One of them, Old Castle Ward (thanks for JacMac30 for finding these), features several buildings as part of the complex which fits McCausland’s report.