By now most have heard the tale of Tuckers Kobdalds where the kobalds on level one were some of the meanest and straight up cleverest opponents in the dungeon. I have always felt this should be the norm and not the exception. Kobalds in the world of Aeril are as a race isolationists, preferring to be left alone, yet are devastatingly clever in particularly in the art of guerrilla warfare and trap making. They are also exceptionally territorial and do not take kindly to uninvited guests. That is not to say they cannot be reasoned with but you better pay attention, bring gifts of food or treasure, and most of all be respectful.
Kobald lairs tend to have several warning signs that can clue you in that you are entering their turf, usually skulls on poles, strange markings on trees, cavern walls, or large boulders. Trust nothing you see in kobald territory else be prepared to suffer dire consequences. Below are traps used at various times in warfare throughout history that I could see kobalds using or devising in the protection of their home and property. At the bottom I will also be providing links for those who would like to read up on history themselves and possibly expand on these ideas a bit.
One of the simplest traps and can be used pretty much anywhere is the placement of venomous snakes tied to a branch, over a doorway, or any where some one not paying attention can walk right into. The snake being tied up for several days and most likely hungry and angry will likely lash out at anything or any one stumbling into it. Viscous and effective.
Snakes inside bags, packs, and chests
Continuing with the use of natural biological weapons, snakes as well as spiders, centipedes, or any other naturally occurring venomous creature works well as a trap or deterrent to any one snooping through a kobald's treasure.
Kobalds are masters of their domain and such are always looking to harvest anything of value from the terrain. Bladders full of swamp gas can make for a terrible time that can sicken those who breath it or if exposed to an open flame create a explosive reaction. Kobalds know this and will setup pressure plates holding back several of these that when triggered release the the stored gas into the area. If flint is available a secondary effect could cause a spark to ignite flammable gases. From here the options and trigger mechanisms are endless. Possible places used are totems, chests, corpses, discarded equipment, etc.
See-Saw Spike Trap
Sometimes it is better to maim then outright kill. Various forms of spiked foot traps where stepped on impales the foot or the spikes are lining the pit at a downward angle to trap the foot and stab at the leg/ankle which may not be as heavily guarded by armor. Almost always poisoned and will cause most who step on them to howl out in pain. The effect is two fold as the victim has now given away their location, and are now an additional burden on those who have to carry or assist their injured comrade. If they are stuck in the trap for a period of time, monsters or a squad of armed kobalds may come scouting to finish off stragglers.
Caves & Tunnels
Kobalds have a natural advantage in small cave and underground networks. Their small stature allows them to move quickly while any one taller then a dwarf or gnome will have to hunker down and move slowly through the area. this provides more opportunities for traps, such as the before mentioned pit spikes, rigging dead ends to entrap the unweary, or just cause a cave in. Most conventional weapons are also useless due to the cramped environment.
Upping the Stakes - Kobalds with Class
Depending on the ruleset your using, kobalds who wish to strike out on their own make amazing rogues/thieves, assassins, and sometimes fighters. Assassin being the most likely in my opinion being able to sneak around effectively, work with various poisons and traps, and understanding the need to strike quick and disappear into the shadows or terrain. A monster race army like orcs, trolls, gnolls, etc would do well to seek the services of a kobald squad of fighters, trap makers, and assassins to soften up enemy ranks, remove upper level personnel, and strike demoralizing fear into the enemy.
This was just a quick exercise at taking a look at our own world history, in this particular case a good majority from the Vietnam war, and find a way to use it for gaming ends. This does not make light of the horror that actually occurred during this time and hopefully it will inspire those who read this to do some research into the topic and learn from the past. Sometimes before moving forward you should look behind you first.