Mies van der Rohe's unrealised design for an amber-glass office tower in the City of London has been revealed in detail for the first time, ahead of an exhibition at the Royal Institute of British Architects next month. The Mansion House Square project was under development for over 20 years but never built,  following a long campaign by Prince Charles – a well-known opponent of modernist architecture. It instead became the site for James Stirling-designed No 1 Poultry.