The Spanish World Cup-winning goalkeeper was happy with his side's display against the European champions but disappointed that it only resulted in a draw after taking the lead
Hosts Madrid conceded two goals before the end of the first half after being 1-0 up in the opening quarter, with a second-half Xabi Alonso strike preventing Jose Mourinho's team from suffering a defeat to their arch-rivals at the Santiago Bernabeu.
After the match, Madrid captain Casillas praised his side's performance, but expressed his disappointment with the final result.
"In the first half we played very well," Casillas told reporters. "I am happy with the game but displeased with the result."
"The game we had in the first-half is the way the team must follow," he continued. "This was a good match to measure the level of Barcelona and Real Madrid, but the season is just starting.
"It does not say much, but we showed we have the team to stand up to Barcelona."
The Supercopa second leg between the two Spanish giants will be played at Camp Nou on Wednesday.