To get into Asan you can get a train to Onyangoncheon station. This will land you in the middle of the city centre. If you're looking for hotsprings then there is a very large hotel spa within a 5 minute walk from the station.
You can take the bus to the city too. Regular buses run from neighbouring city, Cheonan and also from Seoul. The bus terminal is slightly out of the city centre. A taxi downtown Shi-nae will cost you 2000Korean won. You could also get a city bus 420, 410 which will take you to the train station.
Asan also known as Onyang can be easily reached from Cheonan 30minutes by bus or 15 minutes by train and Seoul 90 minutes by slow train or 2 hours by bus.
The closest KTX bullet train station is Asan. This station is about a 20 minute drive from the city centre. REgular trains connect the station to Onyangoncheon throughtout the day. Alternatively, a taxi will cost you about 7000won to get you into the city centre.
