Istanbul's ancient city walls were first built by Constantine the great, and were originally 26 km long, but today just 7 km remain. In the past these walls haven't been adequately maintained, and now need protection quite urgently. It's thought if they were properly restored they could prove to be quite the tourist attraction.
However historians are anxious to ensure the project is authentic, something they believe is only possible if the original stones are kept during the restoration project. Apparently an earlier restoration project in the 1980s to restore parts of the city walls saw some areas completely rebuilt which destroyed their integrity. At the time these restoration projects weren't part of a well-planned program, and different parts of the walls were restored by different experts.
Now experts are looking at ways of strengthening and protecting the walls, and there are plans to dig around the areas surrounding walls in an effort to try to find more original stones for use in the restoration work.
Experts believe it isn't always necessary to fill in gaps in the walls, and argue they should be largely left as they are, showing their ancient heritage. In 2005 there was an international workshop to discuss the restoration of the city walls, and at the time a professor thought the best way forward was to have a maintenance unit continually working on protecting the ancient walls.
Apparently Istanbul Metropolitan Municipality has been working on plans for a restoration project for these city walls, but hasn't yet made the details public. It also appears as if restoration experts and historians haven't yet been informed or consulted about this matter.