We took bus 26 from Cardiff central bus station (£4.20 return ticket). On our way back we went to the Caerphilly Bus station and took bus A back to Cardiff (or bus B, take which one comes first). Buses A & B make different route than bus 26 but all of them end at Cardiff. On Sundays only bus 26 is available.
You can also use the train from Cardiff. The train station in Caerphilly is next to bus station (both of them are located in the city center)
To be honest, Caerphiily is a small place and if you plan on staying in and around the main town then you have no need for any mechanical transport i.e. car or bus. Everything you need is within walking distance. You can walk from the top of town to the bottom (known as the Pic) in 10 minutes and that's generous.
If you plan on venturing further afield then it would be advisable to hire a car or use the public transport. The public transport has improved in recent years with plenty of connections to and from Cardiff. The only day you would struggle is a Sunday as the service is limited. The trains run frequently seven days a week.
Many tourists enjoy hiring bikes (available locally from the tourist board) and although there are plenty of hills around it is a great way to see the local countryside.
Caerphilly is easy to reach from Cardiff. Trains leave Cardiff Central or Cardiff Queen Street every 30 minutes, Monday -Saturday. The fare is about 3GBP return. The station is about 1 km from the castle.
Buses leave the Bus Station A, B , 26 from Cardiff.
By road from Cardiff on A470 and A469; from Newport A468.
The bus dropped us in front of the castle so we just walked inside.
Later we walked a bit in the town, we saw a lot of local buses (probably connecting the towns with other towns). The castle is located in the city center anyway so if you have some extra time you can check this town easily.
The train and bus stations are in walking distance from the castle too (5-6’ away). We walked there to take the bus back to Cardiff.
The best way to travel along Wales is renting a car from the airport if flying to Cardiff.
We had lots of troubles to rent a car, as most of the rental cars companies close on weekends at the airport, strange isn’t it?
Finally we could book online a economy car (Fiat punto, nice and brand new) and returned at Sunday 5,00 am with Hertz.
The train was the fastest and cheapest option we found to go to Caerphilly from nearby Cardiff, faster and more comfortable than a bus, so if you're in the area head by Cardiff Central station give it a shot.
Most people travel to Caerphilly from the Welsh capital, Cardiff. Getting to Caerphilly is quite easy. There is a direct train from Cardiff and there is also a bus from the town centre. Your best bet is to make your way down to Cardiff central train station or Cardiff central bus station (bottom of St Mary Street) and have a look at cost and times etc. All in all, getting to Caerphilly shouldn't cost you more then £10 or so pounds for a day return.
For once, the wonderful UK signs pointed me in the wrong direction and I wasted a bit of time walking the wrong way to the castle.
When you get off the train, go straight down the hill and you can't miss the ruins. Don't pay any attention to the signpost that had, when I was there, the arrow pointing to the right.