Possibly originally called Castell Penweddig, this is a 5m high castle mound with a double bailey and two attached rectangular baileys. It was built by Walter de Bec sometime after 1110, and destroyed by the warlike Princes Owain and Cadwaladr in 1136. It was subsequently rebuilt, but captured by another set of belligerent Princes, Maredudd and Rhys, in 1151. It is a remarkably well-preserved mound, entirely wild - and on private land, but a public footpath goes very close.
St. Michael's Church
There's a local legend that the original builders of the church tried to erect it a mile or so away from its current location, and that it was to be dedicated to a different saint, St. Ffraid. Each time they built up the walls they had fallen down by the next morning, which must have irritated their foreman greatly. Eventually a voice was heard telling them that it should be built where it is today and dedicated instead to St. Michael - instructions that they obeyed, and apparently all went well thereafter. In the grounds of the church is an ancient yew tree, estimated as being 2000 years old, and an information board states that it is thought that in the Middle Ages lightning struck the tree, splitting the trunk into the three sections present today. There's a restored Holy Well nearby, which has the tradition of being a healing well.
Plynlimon and Hafan Tramway
This was a 2'3" gauge line connecting Llandre (Llanfihangel) mainline railway station via Talybont to Hafan Quarry way up towards today's Nant y Moch Reservoir. It is stated elsewhere to have been remarkably unsuccessful, and only ran for 18 months or so in the late 1890s. See "Plynlimon and Hafan Tramway" by E. A. Wade and "Narrow Gauge Railways in Mid Wales" by J.I.C. Boyd (the latter I've not read yet).
Pictures of the Hafan Tramway at: http://www.sbrobinson.pwp.blueyonder.co.uk/plynlimon.html