I think there is enough information on wiki in Trading article, to be honest, including the tunnels with drawbridges option. I used the tunnel with drawbridges as walls at the edge in v.0.31 and some newer versions, but in recent forts I don't care. The invaders come in the beginning of the month, while the caravan comes and goes in the middle, so they are separated chronologically. If there's siege, the caravan doesn't come, and the diplomat comes next season, often with the diplomat from the other race. It's too rare to have caravan destroyed by a siege to care, in my opinion. Main problem is removing corpses and body parts, so they don't get spooked...
The only thing I do is separating the way for walkers and for caravans in my fortress, where the depot is located. I use the trick with a tunnel accessible only to wagons, so they don't have to try to path through the traps (which they can't), while the walking merchants and security walk through traps as normal.
And for controlling trees - either floors or, in further places, just disabled stockpiles.
Still, as far as I know, for people who care, the tunnel is one of the most common methods to make traders safer. And a bridge at the edge of the map is an easy way to stop invaders spawning at the tunnel entrance (the three bridges forming the walls are always up, the fourth, at the edge, is down when the caravan is expected to come, or when it needs to leave). By the way, there's no need to make this bridge only 3 tiles wide. I used to make it 10 tiles wide, so they have more space for manoeuvres.