"but a portcullis was let down on Owain"
Portcullis
Creative Commons Attribution Share AlikePortcullis - Credit: Tschai-Pi

A portcullis is a defensive structure found at the entrance to medieval castles. It consists of a latticed grille of wood or iron which could be lowered at a moment’s notice in a crisis.

Some castles had both an inner and an outer portcullis. Lowering both meant that unwanted visitors could be trapped in the space between the two. The picture below shows the double portcullis at Warwick Castle, England.