One of the aspects of megadungeon play is that the low-level areas are restockable, so that new low-level characters can always enter.When did you run it? For the 5e re-release that is no longer true. There is a series of low level adventures with the Zelkor's Ferry supplement. The Mouth of Doom can be run with (very careful) low-level characters. Castle Calaelen offers low level adventure and the possibility or creating a stronghold/home base for the party. The party can certainly level up to high tier one and perhaps low tier 2 before setting foot in RA proper.
We started at level 0. Plenty to do at low levels.