Cahill put in a strong performance to help his team secure a 0-0 draw in the first leg of the Champions League semi-final at the Vicente Calderon, and the England international has admitted that they travelled to Spain with a gameplan.
The 28-year-old told UEFA.com: "We came here with a gameplan. Our plan worked in the end and we are pleased with the draw. We nullified their threat by the way that we played.
"I still think that they caused us some problems but – speaking from a defensive point of view – the clean sheet was big for us.
"Our coach was pleased because the tactics that he put together came off. We defended here more than we usually do; we were more compact than normal."
The second leg will take place at Stamford Bridge next Wednesday.