Do enemies always spawn on ground floor? If I build vertically, will I at least be assured that they will never spawn inside my tower like forteress?

*bump* anyone knows?

You can't 100% wall off a settlement to prevent the raids. They will spawn inside your walls and even then if you build a base-level and build up from that, they will still spawn at the bottom floor (this is assuming you walled off the entire settlement). I've seen a few people turn Hangman's Ally and the island settlement into giant fortresses and they still get raided.

So far, no - they always start on the ground.

Makes sense - the game would have to have spawn points coded in, and Beth wouldn't bother putting them in elevated positions simply because if we don't build there, the bad guys would just spawn in falling.

I have a fully elevated fort at Sanctuary - the only thing on the ground are farms and a Brahmin, and the whole structure is ringed with HMG and Laser Turrets hanging down (12, plus 4 more on the gate). During raids, all I have to do is show up - the Turrets take care of everything, since the raiders have to go thru an open crossfire from multiple Turrets to get to anything wreckable. So far, no raider has set foot in my actual base (and my entrance is a killzone - I'm just about to add a Shotgun turret facing the door - nothing is getting in there).

TL;DR: Nope, ground only, and turrets > walls. Less stuff on the ground, more field of fire for turrets. Elevated base good.