At the Pantheon you'll see one of Rome's best-preserved ancient buildings up close: the huge coffered dome with its oculus is the showpiece, and the interior is compact so plan on 20 to 40 minutes unless you want to linger. It is an active church, so dress and behave respectfully, and expect short pauses or restricted access during services. Entry is usually free but there is often a security queue and occasional ticketing for special visits, so go early or late to avoid the biggest crowds, travel light because bags may be checked, and keep your camera ready for great interior shots without flash.