Trencin has small historical city centre below the famous Trencin castle. The castle in itself is the number one reason to visit Trencin.
In December there is a nice little Christmas market in the old town centre, which was a great place to mix with the locals. My main criticism of Trencin is that there are not enough Slovak restaurants in the city centre. They are far outnumbered by pizzerias. Other than that it is a great place to stay for one or two nights to see the castle and just chill out for a couple of days.