From the Introduction:
The Castle Triskelion is a "mega dungeon", a sprawling multi-level complex which an entire campaign can be based around. This Introduction explains the basics of what this is and what the area looks like.
The castle may be placed in any adventure setting and modified as each dungeon master sees fit.
Publication of the Castle Triskelion is an on-going, multi-document process that is likely to go on for years of real time. Each level, above and below ground, will be fleshed out, and each level will be a separate publication. As time goes on some sections may be "re-stocked", that is to say, a new set of encounters and items for that level may be published, which will address the idea that adventurers may clear the monsters from a section, leave, and when they come back something else has moved into that space. This provides versatility and flexibility to the DM.
Over time, new monsters and items will appear, and additional "new material" documents will be published, containing that new material.
The introduction of each new section of the megadungeon will include a list of the publications required to run that section. Note that all Triskelion documents are free publications, and will continue to be free.