Stage 18 : Merthyr Tydfill (Rhyd y Car Lesiure Centre) to Abercynon (Navigation Park), 9.1 miles
The official Stage Route Map can be viewed here (as a pdf file).
The stage route can also be viewed here on Gmaps pedometer which includes elevation details, although it must be stressed that the distances and altitude calculated by Gmaps are approximate and not race measured.
Race marshalling and other instructions for this and indeed all 20 stages of the relay can be seen here in the 'Castles Reference Pack'. This quite a large file though (2.9 Mb !).
Start: Rhyd-y-Car Leisure Centre (Grid Ref: SO050055).
Start time: 13:40 pm
Finish: Navigation Park Indistrial Area (Grid Ref: ST082945)