The proposed EU-US trade deal TTIP (Transatlantic Trade and Investment Partnership) has been garnering heated criticism from activist groups across Europe due to concerns that it will lead to lifting certain environmental regulations and hurt the European economy.
The twin KIO Towers where the Madrid demonstration took place are also known as the Gate of Europe towers and were designed by American architects. They were also the world's first inclined skyscrapers, with an incline of 15 degrees.
At a little before 7am on Tuesday morning, the activists began the climb, police and firefighters arriving shortly thereafter to check on the safety of the demonstrators, the group said in a statement.
"The moment when you begin and stay suspended from the cord is the scariest moment," said climber Juanma López, 45, from Almería in a statement. "You lose firm ground. You start to think that everything is fine and about your situation. We were working very quickly and under a lot of pressure. When you see that everything is going well, you relax."
By noon the banner had been completely revealed and by 6pm, 11 hours after they began, the climbers were back on the ground to speak to police, though no arrests were made.
"This trade agreement is supposed to coordinate the reduction of regulations for both blocs, which for the EU would weaken quality controls and the safety of imported products," Greenpeace Spain wrote in a statement.
"The term 'free trade' is putting at risk the standards of quality and important safety measures for agricultural production, food supplies and the health of the ecosystems."
But it's not just Greenpeace who have voiced criticism of the TTIP deal: 146 Spanish towns have declared themselves against the trade agreement, according to El Pais
This includes Madrid, Barcelona, Seville and Valencia. Another 270 Spanish ecological organizations have also come out against the proposed deal.
Thousands have also taken to the streets to protest the deal in places like Germany
Supporters of the deal, however, argue that it would boost both the US and EU economies by some €100 billion each, and create jobs by "helping cut red tape" that companies face, according to the European Commission.