Book your place on the tour of Bastion passages. I was lucky to get a spot the for morning I called. It's only one long corridor, but presented in a fresh and interesting way, firstly by a funny...
An enjoyable attraction, not far from Freedom Square. I wouldn't bother with the tunnels, tour too long and a little boring, just too the castle part.
Good museum to discover about Tallinn's history. Guide was very good and very interesting. It is also good for families.