You can access Prifddinas by talking to the Elf Hermit at home when you have 2250 total, or if you have the perk you can use the ;;prif command.
In this beautifully complex city there are plenty of amazing things to do that involve Mid-High level skilling.
In Prifdinnas there are 8 different discricts. Each district houses a clan that focuses on different aspects of skilling. (Eg, Trahaearn focuses on mining and smithing; Hefin focuses on Agility and Prayer.)
A good portion of people get lost in this city because of it's monstrous size and complex structures. The picture below w