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[Oaktree]

Control and monitoring of the surface area of an embark is always a particular concern.  Caravan wagons have requirements for available path width,  thieves and ambush teams sneak around, and there is also the local wildlife.  And if you are surface farming or pasturing to any extent you might desire to use surface area for this as well.

Biomes have a large effect on this.  Trees will block access, and thus the concentration and growth rates will make an area increasingly "hostile" to wagons unless cut back.  The same cover affects use of ranged weapons as well.  Rivers and pools affect this as well since they can interfere with pathing, formation of defenses, and add the hazard of ice in climates that have freeze/thaw cycles.

Surface area can be fully secured by walling and roofing it.  This is labor and material intensive, but worth the cost generally.  (I generally have a 2-3 story building near a secondary gate.  Used for work parties, migrants, non-wagon caravans, and serves as a squad barracks.)

Walls only is fairly secure, but can be attacked by flyers.  This threat varies by biome; with some flyers being much more bothersome than others.  Walls require materials, and forts are no longer swimming in extra rock like they used to.

Dry ditch or magma moat is secure from non-ranged ground attack.  And with high-skill miners stretches of dry ditch can be dug fairly rapidly. 

Both wall and ditch are useful for channeling visitors towards specific areas since they block pathing for ground units.  So one solution for controlling the surface of the embark is extensive outwork construction to create chokepoints.  Included with this is the concept of using extensive tunnels and exterior entrances in order to get visitors underground as soon as possible; which puts them into a much more controlled and constrained environment as soon as possible.

For terrain with tree issues one solution is paved roads.  If built 3 tiles or more wider these guarantee a valid wagon path through wooded or otherwise blocked terrain.  This saves on tree cutting expeditions - though if you're clear cutting anyways...
(Note that a wagon will not use the road if there is a more direct path via the woods.)

Monitoring is another issue.  Bridges, doors, etc. need to be unlocked and open for use by expected visitors.  Plus certain thieves pick locks anyways.  There are a number of solutions for this: guard animals, traps of various complexity, and patrols.

Kobold thieves require guard animals or patrols to detect unless you set up some sort of "bucket of water" trapped door on the shortest path in.  Goblin thieves are stealthy, but traps work on them.  You might only see the corpse or bits, but they didn't get through.  They are also detected by guard animals and patrols.  Goblin ambush teams are detected by guard animals, patrols, and triggering traps. 

Guard animals can be placed by restraint (chain) or pasture zone.  War dogs at doors or in tunnels is a common (and early emplacement) of this concept.  More advanced monitoring buildings using birds or cats can be built.  Animal watchtowers use glass windows to allow observation.   Another solution for a chokepoint is to have animals pastured above it on grates to observe whatever is passing below.  Animals can also be placed as "bait" in various places to draw attackers into traps, or simply to serve as early warning when they are attacked and killed.

Simple traps (cage, weapon, stone-fall) serve multiple purposes.  In a chokepoint they soften up or deter attacks.  They can detect an ambush team who stumbles into one.  Besides being a deterrent and monitoring device a trap can also now be used for wagon traffic control since wagons will not cross them. 
Note that for cage and stone-fall traps there are jobs generated to reload them after triggering.  So for outer areas weapon traps may be preferred, or the trap forbidden until the area is safe for civilians to transit.

Complex traps are the meat and potatoes of chokepoint defense, and probably too labor intensive to scatter about randomly.  With mechanisms, time, and other materials, semi- to fully- automated death traps can be constructed.  Invading goblins can be minced, washed, melted, burst, atomized or otherwise disposed of in some combination of entertaining methods.

Militia patrolling is dependent on the quality and quantity of the military dwarves available.  Crossbow bunkers are fairly secure and can be built to a number of designs.  Foot-on-ground patrols will see more, but run the risk of being overwhelmed by superior forces or the onslaught of an unexpected sample of local wildlife. 

So, given this, what are the various overlords preferred methods for surface control?

I'm partial to extended tunnels from a central location.  Each tunnel is sealable at the exit and interior point with drawbridges to control access.  The exterior exits have a paved road running to the closest embark border.  I was originally doing animal watchtowers for monitoring, but am favoring the "overhead grate" method currently as being simpler to build.

The tunnel chokepoints tend to lead into a mix of goblin grinder, magma dodge-me, and crossbow bunker.  Two sets so that one can be "open" while the other is closed to be cleaned out of corpses and goblinite.  With the new wagon pathing there are 4-wide by-pass tunnels with drawbridges for the wagons to use to go around the trap complex. 

And I am also thinking of putting out a series of one weapon weapon traps to serve as ambush detection and to "push" wagons towards the tunnel entrances.

[Urist Da Vinci]

You can also channel a hole in the ground, put a grate on top, and pit an animal through the closed grate into the spot to act as a watchpoint from below the grate. A series of these in a 1-tile-wide tunnel is very likely to spot any stealthed intruders. Adding a replacement animal involves simply pitting another in the appropriate spot.

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Note that for cage and stone-fall traps there are jobs generated to reload them after triggering.  So for outer areas weapon traps may be preferred, or the trap forbidden until the area is safe for civilians to transit.
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And I am also thinking of putting out a series of one weapon weapon traps to serve as ambush detection and to "push" wagons towards the tunnel entrances.

Weapon traps also generate "clean trap" jobs. Instead, you can build any trap, and forbid it after completion to prevent any dwarf from accepting the job and walking into danger.

[wuphonsreach]

I'm in a heavily forested biome at the moment, every year, I order the woodcutters to clear an 11-tile wide "path" back to the nearest edge, to cut down any trees that have grown up.  Which means the wagons / traders always show up at that point in the map.

Eventually (Armok willing), I build a 7-tile wide "wagon path", with walls on both sides and a 5-tile wide paved road running down the middle out to that edge.  It has a few twists and turns to make it a slightly longer path then running straight towards the fortress.  Along with at least 2 lever-activated bridges along the way for control.

Then I build a 2nd wagon path to the opposite side, with a similar design.

[krenshala]

Trees and boulders (natural, not the kind your miners create) block the eight tiles immediately surrounding their position. The wagons only need a clear three-tile wide path in, however, the center tile of the three is the only one that must be completely clear of obstructions.  This means you could have a tree lined three tile wide path that still allows wagons (tree, floor/road x 3, tree).