Complex sentence: Sybil had held Ludlow Castle, an important strategic stronghold for control of the Welsh Borders built in the late 11th century, against Stephen in 1139, but surrendered after a siege, and Stephen then married the widow to someone he felt was trustworthy.
Split into simpler sentences: Sybil had held Ludlow Castle against Stephen in 1139, but surrendered after a siege. Ludlow was an important strategic stronghold for control of the Welsh Borders, and Stephen decided to marry Pain's widow to someone he felt was trustworthy.