Valheim Guides
A Lighthouse is one of the more important vanity buildings that you can create for your port base in Valheim. The first decision you'll have to make before starting your Lighthouse is do you want it to be square or round? As of the date I wrote this guide Valheim did not have any circular building pieces; the only way to build a round structure is by snapping rectangular pieces together until you wrap them around to touch again.
If you're a perfectionist when it comes to building like I am I don't recommend you build a circular structure. It will look and work fine but it'll be very imperfect in parts and feel slapped together. You'll never be able to get the roof perfect, the floor will have clipping issues and small gaps you can't fill and the list goes on. Many of these imperfections you can hide with rugs, stairs, a chest or building whatever else ontop of it or to block the line of sight so they may not be a big deal to you. However, I feel that part of my job as a guide writer is to point these things out ahead of time so that you know.
The last imperfection I will point out is the stairs. When you're building a circular lighthouse it'll be much more difficult to properly build stairs that lead up to the top.