Depends on the end result you want. You can build using elevated pillars as 'foundation' points and save quite a bit of stone, plus can make a dock at least partially underneath - not sure about mast height though especially during storms.

On the positive side, if you use the highest 'natural' points as your pillar locations, then the base will be nigh impregnable to ground based mobs.

On the negative side, you'd need very large pillars to raise crops. Also castle architecture might be challenging to make it look good. There's also a bit of background work needed before you can really get started - for instance you'll want to make a small pillar (perhaps will be used later) just to get a sense of the terrain tile orientation. Also will want to build framework and scaffolding to get everything lined up right and be able to build at 8+ meters above ground level.

PS: might want to make a duplicate world using same seed and try different ideas out in creative mode before going back to your Viking's world and doing it for real in survival mode.