Kaunas Castle is considered the oldest castle in Lithuania. About a third of the castle survives today.
Info provided by McMaster:
Kaunas Castle is located in Kaunas, Lithuania. Archeological evidence suggests that it was originally built during the mid-14th century, in the Gothic style. Its site is strategic – a rise on the banks of the Nemunas River near its confluence with the Neris River. At the beginning of the 21st century, about one-third of the castle was still standing.
Today the round tower of Kaunas Castle houses an art gallery. The castle is open to tourism, and hosts occasional festivals. Plans have been prepared for a major castle reconstruction project.