After the info/tour of the Queen's Staircase, you can walk up the staircase to visit Fort Fincastle and the water tower. There is also a straw market and a restaurant near the market and tower. You can't assess the water tower now, but you can get some good pics of the island and ships in port at the top of the fort. It costs $1 to enter the fort, but it's worth the views.
You can also tip a guide to tell you the history of the fort. Since we herd a lot of it from our guide for the Queen's Staircase, we skipped the fort tour. Good alternative to the typical cruise ship tours and you can do this tour on your own.
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