What are the requirements to enter Prifddinas?

To enter Prifddinas(Elf City), there is only the quest requirement of finishing Plague's End. Plague's End does have requirements in order to start it.

Quest requirements:

- Making History
    - The Restless Ghost
    - Priest in Peril
- Catapult Construction
- Within the Light
    - Mourning's Ends Part II
        - Mourning's Ends Part I
            - Big Chompy Bird Hunting
            - Sheep Herder
            - Roving Elves
                - Waterfall Quest
                - Regicide
                    - Underground Pass
                        - Biohazard
                            - Plague City

The level requirements for Plague's End are:

75 Construction
75 Dungeoneering
75 Agility
75 Crafting
75 Herblore
75 Mining
75 Prayer
75 Summoning

Once you have finished Plague's End you will have access to Prifddinas! Just head to the gates in Tirannwn, and enjoy the beautiful city.

Plague's End Wiki Source