You can reclaim IIRC, however the place will have some baddies there. Lost a fortress due to a tantrum spiral.
I abandoned the fortress in the end.
Reclaimed it later. But there were some creatures there, lost some soldiers.
Think I cleaned it up, but lost it again due to tantrum spiral.
That was in version 40d however, before v0.3.
Don't know about adventurers retiring living on their own. I usually often just die.
And yes, I think there are some sort of travelling going on. You can get besieged by enemies, depends on what civilization you have nearby. Usually it's goblins.
And watch out for kobolds, thieving bastards.
Sometimes there will be Fun! arriving, haven't had it myself but I've read some stuff about some nasty enemies coming into ones area.
Haven't played the latest version all that much, it's so difficult to get into it sometimes. It has kinda like a learning cliff face difficulty to it.
It's rewarding though when you've learned the basics and begin to understand how it works.
Had most of my experience with 40d. They changed so much when 0.3 came around.
Here's a link to the forums
http://www.bay12forums.com/smf/
And as they say:
Losing is fun!