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.

The trial focuses on Epic’s contention that Apple has turned its store into a price-gouging vehicle that not only reaps a 15 per cent to 30 per cent commission from in-app transactions, but blocks apps from offering other payment alternatives. That ban extends to showing a link that would open a web page offering commission-free ways to pay for subscriptions, in-game items and the like.