Husband and I are wanting to spend about 4 nights in Wales in late May 2018, but are looking for a unique experience such as staying in a more rustic or medieval type lodging, or just in an area with castles and more rustic/medieval type architecture and have it feel like we're back in time. We would be flying either from London to Wales or Wales to London depending on the other part of our trip, and it appears Cardiff might be the best place to fly into...just not sure that's the best part of Wales to actually visit to get that 'old timey' experience? We also won't have time to do the whole rental car thing, we would probably do that if we were staying longer but 4 nights isn't much...which presents the need for public transport of some kind once in Wales (unless you HAVE to rent a car to get anywhere?). I realize we might be asking a lot...just not very knowledgable about our options at this point.
South Wales is easily accessible from London by train or coach (bus).
Here is pretty good list of picturesque towns that might interest you. I can personally vouch for Chepstow, Pembroke, and St David’s (no castle but a cathedral and bishop’s palace ruins). Caerphilly Castle is nice but the town is probably not what you are looking for. The rest of this list I’m not familiar with. Keep in mind these will also be touristy as well with the accompanying trinket shops, etc.
If it were me I’d head to St David’s.
You may like to visit this castle on the northern edge of Cardiff:>http://cadw.gov.wales/daysout/castell-coch/?lang=en
(It is Stagecoach bus 132 which can be caught from the bus stop on the east side of Cardiff Castle - get off at Tongwynlais - ask driver to tell you the nearest stop to the Castell Coch - you will need to walk for about 14 minutes up a hill).
Whilst Castell Coch is a VIctorian reproduction of a medieval Castle, Caerphilly Castle is the real thing - though the town of Caerphilly is a bit of a ‘dump’. Best ay to get to Caerphilly from Cardiff is the train.http://cadw.gov.wales/daysout/caerphilly-castle/?lang=en
If you want Welsh medieval style entertainment - try and attend a banquet in Cardiff Castle:> http://www.cardiffcastle.com/welsh-banquets/ (Please note that many of the pubs in the city centre tend to be aimed at young people - who tend to drink too much. A lot of restaurants are found in the Mill Lane area and along the waterfront at Mermaid Quay, Cardiff Bay = 1 mile south of the centre - use buses).
When in Cardiff - be sure to visit > https://museum.wales/stfagans/
By just going to Cardiff, you have not seen Wales.
Initially it sounded like you really wanted to experience life as it was - cesspit in the house, open fire for cooking, smoke in the rafters, "interesting" smells, livestock around, straw mattresses, rope beds, no heating or glass in the windows, etc.
I think perhaps that that isn't what you meant. Or is it?
If you go to South Wales you may be interested in St Fagans museum. They have transported and relocated a number of historic buildings from across Wales so that it is a living walk through museum. Free entry, accessible by bus from Cardiff centre about 15minutes.
It's no longer possible to fly London to Cardiff but train is 2 hours. Book tickets in advance and avoid peak times otherwise it's a very expensive train route.
You'll find plenty of 'rustic' accommodation around but more often than not that phrase will be used to describe poorly equipped and maintained properties. I can't think of anywhere that with give you a medieval experience or one that will make you feel like you're back in time, people live in the towns and villages and modern life goes on. There are plenty of old buildings that you can stay in however the proprietors will be aiming to provide you with a modern, comfortable stay. If you want to experience something medieval then Warwick Castle will be suitable, an historic castle but one that is operated more along the lines of a theme park.