Charles's charity, the Prince's Regeneration Trust, has bought the Grade A-listed Kinloch Castle on the island of Rum in Scotland.
The organisation is drawing up proposals to renovate the rundown building and add a series of eco-additions to its fabric.
Completed in 1901, Kinloch Castle was originally the home of industrialist Sir George Bullough. In 1958 his wife sold the property, which would today cost £15 million to build, to what is now Scottish Natural Heritage (SNH).
The Prince and the Duchess of Cornwall visited the site in June, and it is understood that Charles has helped draw up plans for a 'green castle'.
A spokesman for Clarence House confirmed Charles' involvement in the project. He said: 'Prince Charles is working with SNH to put in a plan for Kinloch Castle.'
He added that the Prince always endeavoured to be environmentally friendly and any future restoration project would have 'green elements' to it. These would include solar panels and an independent power supply.
Work is in progress on the second phase of feasibility studies and other options which will be considered.