This was inspired by Sting's divorce. The pain he felt when he couldn't make his first marriage work led him to write some of his biggest hits, including "Every Breath You Take" and "King of Pain". Sting wrote the song in the studio in Barbados in 1985. The song features a Branford Marsalis sax solo,. The inspiration for this song was the pain he felt from the failure of his first marriage. In a Musician magazine interview later that year, he said:

"'Fortress' is about appeasement, about trying to bridge the gaps between individuals. The central image is a minefield that you've laid around this other person to try and protect them. Then you realise that you have to walk back through it. I think it's one of the best choruses I've ever written."

During one of Sting's first performances of the song in concert in Paris, his crew lowered a tiny fortress onto the stage in a parody of the similar Stonehenge scene from the film This Is Spinal Tap.

The song was also released as a single, and reached #8 and #49 on the U.S. and U.K. singles charts, respectively. It also reached #1 for two weeks on the Billboard Top Rock Tracks chart, becoming his second consecutive #1 hit on this chart.

During rehearsals for this tour, Sting had a camera crew along to make a documentary called Bring on the Night.

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Wikipedia