They offered to build a viewing platform atop the 69-foot high tower, from where visitors could view the reserve, with its mixed-growth forests, lakes, rare birds and amphibians.

“We realized we had something very special, and that it could be a regional highlight,” Ms. Phillips said. “We didn’t see it as living in a tourist attraction; it was more to do with creating something special within the Unesco-protected area.”

In 2003, permission was granted, and the couple paid 75,000 euros ($100,000 at $1.30 to the euro) for about three acres of land surrounding the tower. Because the tower is designated a historic landmark, the town decided not to sell it, but rather to grant them a 99-year lease for 500 euros (about $670) a year. (They can sell it at any point within that period.)

That same year, they hired Frank Meilchen, an architect based in Berlin, to help them with the conversion. “I think it is one of these childhood dreams of living like a knight,” he said of the project. “It had a romantic association.”

For Mr. Meilchen, the biggest challenge was removing the huge concrete water tank inside the tower. Workers cut it apart using a diamond saw, and a crane operator drew each segment out the window, lowering it to the ground. The operation took a month. “It was a completely modern ballet,” he said.

Mr. Meilchen then constructed another, weight-bearing tower within the original that holds up the new floors where Ms. Phillips and Mr. Hurding live now. He also built another tower alongside the original one, windowed in red glass panels, to house the elevator that brings visitors to the viewing platform.

The renovation cost about 600,000 euros ($800,000) and took about eight months to complete. The European Union contributed 270,000 euros ($360,000) toward the elevator, to make the site handicapped-accessible.