Apple CEO Tim Cook leaves the Ronald V. Dellums building in Oakland, Calif., after testifying in a federal court case brought by Epic Games on Friday, May 21, 2021. Epic, maker of the video game Fortnite, charges that Apple has transformed its App Store into an illegal monopoly.

SAN RAMON, Calif. — Apple CEO Tim Cook described the company's ironclad control over its mobile app store as the best way to serve and protect iPhone users, but he faced tough questions about competition issues from a judge Friday about allegations he oversees an illegal monopoly.

The rare courtroom appearance by one of the world's best-known executives came during the closing phase of a three-week trial revolving around an antitrust case brought by Epic Games, maker of the popular video game Fortnite.

Epic is trying to topple the so-called “walled garden” for iPhone and iPad apps that welcomes users and developers while locking out competition. Created by Apple co-founder Steve Jobs a year after the iPhone's 2007 debut, the App Store has become a key revenue source for Apple, helping power the company to a $57 billion profit in its last fiscal year.