We had 3 days in Old Town and spent pretty much one of those days at this Museum. You have 3 options: Tower entry, Bastion entry, or a combo. We somehow lucked out and got onto the last English Bastion tour without a reservation (but as, others said....make a reservation as tours are limited to only 20 people at a time). I don't think you need to be paranoid about the reservation issue by making it 30 days in advance, from home. But do make it when you arrive, especially if you need one in a specific language.
So...we had to wait a few hours for our Bastion tour and decided to do the combo ticket and discovered that the Tower was really REALLY good as well. It's a thoughtfully designed museum of Estonian history with dioramas and cool displays that walk you through the unbelievable history that Estonia has had to endure over the centuries and up until modern day! It's a wonder they've managed to maintain their culture through out all of the invasions they've had to endure. You will not appreciate this until you visit this museum.
Word of caution though--You need to climb the original, uneven, mismatched height, twisting stone stairwells of the tower to get to the different floors of the museum. Children will LOVE it. The FIT will love it. If you are physically challenged, you will despise it. I looked for lift options and did not see any (nor could see how they could be put in there).
Now, for the Bastion Tunnels Tour--If you can ignore some of the plastic mannequins they placed in some of the areas to "set" the feel for the different time eras the tunnels represented, and listen to the guide as they inform you of the history of the tunnel as you proceed through the tour, it's really quite astonishing. There is a vast network of these underground passageways under Tallinn and the tour actually ends where they are continuing to excavate through more of the length of the system. So, you get to end the tour with a glimpse of archeology in progress.
Oh...as for temperatures. I come from an area of the US where there are some of the best underground cave systems. Average cave temperatures are about 54 degrees F (12 C). Spending time in our Mammoth Cave in Kentucky, that's NOT that BAD....so stop complaining :) If you go in knowing this, bring a sweater. If you forget, they can provide blankets.
It was all a really excellent experience. Well worth the money and time.
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