Site de Castennec gets one star in Michelin guide to Brittany (who use a three start rating system) with the comment “this celtic site became an oppidium and then a fortified Roman camp”. I couldn’t find out any further information on the web.
It is signed off the the D1, the road between Guemene sur Scorff and Plumeliau, near St Nicholas des Eaux. As we were going past we decided to stop and find out what it was. There is a small car park and a sign to the viewpoint. This is a short walk with a few steps to a viewing platform, high above the river Blavet. (It's not really a nature reserve, but this seemed to be the best description in the list of alternatives for attraction.) There are good views across green pastureland and woodland or the other way to St Nicolas des Eaux.
The river here makes a big loop here and the neck of the peninsula is only 100m across at its narrowest point. The peninsula was the site of a fortified Roman Town although there is nothing of it left to see.
It is worth stopping for the views if you are driving past, but not making a special visit.
However there are plenty of other things to do in the area if you want to make a full day. St Nicholas des Eaux is a pretty town on the River Blavet. Close by is Chapelle St Nicodeme whose elegant, open work spire stands above the countryside as a landmark for miles. In September 2011 this was undergoing a major restoration project and a notice on the door said the chapel was open daily in August but only on certain days in September.
Leaving Site de Castennec going north, take the road to Bieuzy and watch out for the sign to St Gildas Chapel at the first left turn. This is a delightful setting down by the river and a nice place for a picnic. The 15thC chapel is built into the side of the massive cliff face we had seen from Site de Castennec.
Many of the small villages in this area repay exploring. We enjoyed Melrand and Quelven.
A bit further away is Poul Fetan, a small 16thC hamlet which has been restored as a typical example of a working 19thc Brittany hamlet and also Village de l’an Mil which is the reconstruction of a medieval village based on the excavated ruins of Lann Gouh near by which was deserted after the black death. Both of these are well worth visiting.
We also liked the tiny hamlet of St Adrian to the south, surrounding its 15thC church with remains of wall and ceiling paintings.
Although I've only given Site de Castennac 5 stars, the area itself deserves considerably more – 8 at least.