The Turku Castle is fantastic! It was definitely the highlight of my visit.
From outside it seems fairly humble, although still cool. Its walls are simple, white, washed out, worn looking. And then you step inside....and the hours just disappear.
It's definitely worth paying the fee to spend time wandering and learning. There are different parts of the castle from different eras - Medieval, Renaissance, and the separate Bailey - and they are all super cool.
There's an exhibition explaining the history of the castle, and it's extremely impressive, spanning hundreds and hundreds of years. This is a true castle, not a pretty palace, but a castle which has been fought over and conquered time after time. The Finns, the Swedes, the Danes, and the Russians have all fought over it and inhabited the castle at some point.
You can watch a timelapse video showing construction and how the castle has changed over time, which is very interesting. And there's even a room where you can dress up in real chainmali and armour - definitely a highlight! Three hours were gone before we knew it. read more