Best Castle in South Wales: Caerphilly, Chepstow, or Other?

If you are driving from Bath to The Cotswolds, then obviously you would be travelling north-east but to go into south Wales, you would be going NW - so, it is a detour and much depends on which part of The Cotswolds you are heading for as they do indeed extend all the way to Bath.

Being that it is a detour, I wold suggest that you simply cross the estuary on the M48 bridge and visit Chepstow Castle. You could then either proceed to The Cotswolds or drive up the beautiful Wye Valley on the A466 and visit TINTERN ABBEY (ruin). Should you do this, then I would continue towards Monmouth and then turn east onto A40 which becomes the M50 which would get you to near Tewkesbury from where you could proceed E to the northern Cotswolds. (Caerphilly is the better castle but that would involve driving into a more congested part of south Wales which has the notorious bottlemeck on the M4 at the Newport tunnel - which the Welsh Government refuse to build a new motorway to the south because they say it is not being ‘green’.
http://www.castlewales.com

Please zoom down on Chepstow on this map and study the one way road pattern to the castle as it can be quite confusing:> https://www.google.co.uk/maps/@51.6396867,-2.6734703,13.4z/data=!5m1!1e1

I agree with James. Chepstow is the shortest deviation from you intended destination. Tintern Abbey is nice. Be aware that just north of Chepstow, at St. Arvans, the A466 is closed until next May or so, up to Tintern. The detour to Tintern is via Trelleck and a narrow road back to the A466. In Tintern, turn right and go a mile to the Abbey. From there, the road to Monmouth is straightforward. Monmouth is nice to visit for an hour or so. There is free parking for an hour in the Waitrose grocery store lot.

Probably the easiest way to Caerphilly castle is exit 32 off the M4, onto the A470, then to Caerphilly road. The road name changes to Nantgarw rd in town. There is a parking lot in the center of town, but is busy and often full. However, when coming into town on Nantgarw rd, turn right onto Crescent rd, that runs behind the castle. There is a parking lot just past where the road momentarily narrows to a lane. Plenty of places to park. Also, if you decide to go to Caerphilly castle, you may as well visit Coch castle since it is right off of the same M4 exit.

Also in the area of Monmouth, there are a number of castle ruins, most are free to enter but not as nice as the ones you mentioned, including Grosmont, Skenfrith, White, and Raglan. The roads between them are narrow, and a couple are very narrow.

I think that the A466 between Chepstow & Tintern is now open again - well, at least Google Maps seems to say so & they are usually accurate. You may like to know that a wonderful viewpoint called ‘Eagle’s Nest’ is just a short detour off this route. Drive through St.Arvans & on the next bend take minor road for half a mile to parking area on R. Then a walk of about 17 minutes up the zig zag path through woods. Where path levels off - viewpoint is just to the R but poorly signposted. (Don't use the lower car park by the A466).https://www.google.co.uk/maps/@51.668509,-2.687965,1786m/data=!3m1!1e3!5m1!1e1

Great that the A466 is open again. That detour was a pain. Had it been open, that overlook James mentions sounds like it would be a nice diversion.

I'm not sure if this castle would work with your drive but Powis Castle was beautiful with impressive gardens. The majority of castles were ruins-Powis is fully furnished and well worth a visit. It was near Welshpool. A really charming area is Llanrwst. The tea room/cafe there Tu Hwnt ir Bont is very good and apparently enjoyed by the Royals.

Caerphilly was the best, but it is close to Cardiff.