In my solo game, I build medium sized house on the top of a hill, surrounded by a garden, a kitchen and my forge. I also got a dock down by the shore. I didn't put any walls, just some fence here and there for decoration purpose. Everything is quite simple, yet neat.

So far, I didn't get any trouble at all. If a raid happen, I go outside, if I am not already, and ennemies come at me to fight, so they leave my house alone. Even if they destroy something, because I didn't go to them quick enough or fight a bit too near of my installations, all the materials are on the ground, ready to be taken back once the raid is over. It take a couple of minutes to repair what have been destroyed after the raid.

I only needed to rebuild a bit one time after a raid (it took me less than 5 minutes). The other times, ennemies were after me so I could easily attract them away from my installations.

They could be useful if you're more into the building aspect and make a big house, with multiples levels and fancy design that takes you hours and hours to put down and you want to take extra care to prevent ennemies to damage it.

All that to say; no, walls are definitively not require. For me, they are more for aesthetics purposes if you want to make a castle or fort, than a real asset in the game.