Llansteffan castle sits on a much older Neolithic promontory fort, proving Llansteffan has been inhabited, and perhaps of strategic importance, since the New Stone Age. The hill's summit can only be reached from one side, and the hill where the castle/fort stands commands the entrance to the River Towy (unlike today, the hill would have been stripped of trees to make foot soldiers vulnerable to attack by archers). The original earthworks can still be seen and were used as part of the modern castle's defence system--the castle proper rests within the earthwork rings.
Llansteffan castle was built by the Normans after 1100 as part of their invasion of Wales.
The castle was captured by Rhys ap Gruffydd in 1146 against the forces of Maurice FitzGerald and his brother William FitzGerald, Lord of Llanstephan and Lord of Emlyn respectively and effectively the leaders of the Norman settlers of the region. The castle was later retaken by the Normans. Llywelyn the Great recaptured the castle for the Welsh in 1215.
The castle was taken and captured by the forces of Owain Glyndwr and held until 1403 when it was taken back by Englishman Sir John Pennes, who was shortly himself captured but back in charge of the castle by 1408. It is rumoured that a secret passage leads from the castle to the nearby Plas Mansion.
As castles go, Llansteffan Castle is fairly simple and somewhat minor, and there is extensive damage done to all sections. Nevertheless, its worth a visit for the magnificent views over the Towy estuary.