Yeah, the ^ signs are traps, the white ones are stone fall traps and the red ones weapon traps. As you can see in the pic, I've put weapon traps all along one side of the chasm, I've since done the same on the other side; the chasm is the main place that I get attacked from (I've not had anything coming out of the river yet) and the traps kill 90% of attackers the moment they jump out. It's very efficient because the bits of monster are just thrown straight back into the chasm
I've since added another two lines of weapon traps to the entrance, each with three weapons that dead enemies dropped (which is a very cost effective way building good traps, since there's not much else to do with captured weapons, and you end up with quite a few).
Aside from traps, I've built my barracks next to the chasm so a fast response is possible if anything gets past (though I don't think they've been needed since I built the traps).
Also, though you can't see them in this pic, I put two ropes on each side of the chasm, and attached war dogs to them, meaning that if anything does get past the traps, then they'll be killed by the dogs. I've seen bloodstains around the dogs so they are doing some work. I also put ropes with dogs attached on either side of of my main entrance, to defend against kobold and goblin thieves that enter via the main gate. In practice, the thieves often just run past them, but the dogs still sometimes take one of it's legs off or something. I usually activate a small squad and send it down the main corridor, after the thief.
With regard to the incredibly elaborate defence systems you often see, they're mostly constructed out of fun and creativity, rather than actual need. Goblin sieges can be quite nasty, but I managed my first one by just sending my 12 soldiers (not all that skilled) just running out towards them. This should have been a suicide mission, since there were about 30 goblins, of which about 15 had crossbows and were firing at my dwarves for all they were worth, but I killed the whole lot of them with only four dead and three wounded.
Efficiency is, quite simply, more important than complex defence systems.
I don't think either dwarves or animals die of old age, though children do grow up, of course. This means you'll need to keep an eye on the population of cats and horses you have in your fort (you can never have too many (war)dogs, of course) of they'll start to slow the game down. The supply of bones, leather and meat that you get from slaughtering excess animals is nice.