If you play without building you will also play without crafting gear, and without a convenient respawn point, and without sleeping at night. Your crafting stuff needs to be sheltered and it can be picky if the wind is blowing the wrong way. But it doesn't need to be a castle. And when you tear things down, or if a troll does it for you, none of your materials are lost if you pick it up within a couple days. (Never lost at all if there's still a workbench in range.)

A ditch with steep sides around the outside and workbenches or campfires spaced regularly inside your base will keep out anything that doesn't fly. Use a simple wood beam as a bridge or leave a jump gap and the local mobs will be stuck outside. I've never had my base destroyed in a raid.

As for solo difficulty goes, it is quite doable. I always play solo. I would recommend reading guides to take down bosses, and for general playing tips. Be vary cautious about new biomes, especially while sailing. Get the proper gear, food, and mead (poison or frost) for the area, keep a functional bed nearby, and sneak around if you're not used to a place yet. Valheim is not too difficult, if you are careful and have some idea of what you are doing.

If you rush around in a biome above your level without the right gear then Odin will not help you.


Beyond all that, it is a fabulous exploration game. The map is different in every playthrough. You might find what you need right away. You might have to search. You might have big flat areas, or you might have ravines all over the place, or you might get a bunch of small islands. The scenery is fantastic, once you get used to the art style. Go look at the screenshots, but don't worry about the castles.

Here is an example of a base when I couldn't be bothered to build big. It's just barely big enough to hold my crafting stuff at the Black Forest level. Outside, are my kiln, smelter, and forge with a simple roof over it.
https://steamcommunity.com/sharedfiles/filedetails/?id=2782681501


And here is where I converted an already existing building to make the fire work without smoking up the inside:
https://steamcommunity.com/sharedfiles/filedetails/?id=2754005745