Jamie McGowan got down on one knee to propose to Bailey Webster in the observatory tower at the 11th century Lincoln Castle after telling her he had arranged a private tour just for them.
Jamie (32) who runs his own media company, said: "I contacted the castle to let them know what I was planning to do and they came up with the idea of cordoning off the tower, which is the tallest on the castle.
"Bailey thought it was part of a private tour when we arrived and the staff were all in on the secret so when we got to the top I got down on one knee and popped the question.
"It was great as she had no clue about it and, of course, she said yes."
As the couple came down from the tower they were greeted by staff and other castle visitors who clapped and cheered.
Jamie, who is a councillor in Hapton with Park and attended the former Towneley High School in Burnley, added: "It was amazing, a really special moment."
The couple are regular visitors to Lincoln after falling in love with the city after 26-year-old Bailey, who is an exams officer and former student at Burnley's Blessed Trinity RC College, accompanied Jamie on a business trip there.
Together for eight years, they met through their mums, who are work pals. The couple, who are now planning an engagement party for family and friends, hope to tie the knot in the summer next year.