Let's start with a passage from the SRD:
Kobold
Small Humanoid (Reptilian)
Hit Dice: 1/2 d8 (2 hp)
Initiative: +1 (Dex)
Speed: 30 ft.
AC: 15 (+1 size, +1 Dex, +1 natural, +2 leather)
Attacks: Halfspear -1 melee; or light crossbow +2 ranged
Damage: Halfspear 1d6-2; or light crossbow 1d8
Face/Reach: 5 ft. by 5 ft./5 ft.
Special Qualities: Darkvision 60 ft., light sensitivity
Saves: Fort +0, Ref +1, Will +2
Abilities: Str 6, Dex 13, Con 11, Int 10, Wis 10, Cha 10
Skills: Craft (trapmaking) +2, Hide +8, Listen +2, Move Silently +4, Search +2, Spot +2
Feats: Alertness
Climate/Terrain: Any forest and underground
Organization: Gang (4-9), band (10-100 plus 100% noncombatants plus 1 3rd-level sergeant per 20 adults and 1 leader of
4th-6th level), warband (10-24 plus 2-4 dire weasels), tribe (40-400 plus 1 3rd-level sergeant per 20 adults, 1 or 2 lieutenants of 4th or 5th level, 1 leader of 6th-8th level,and 5-8 dire weasels)
Challenge Rating: 1/6
Treasure: Standard
Alignment: Usually lawful evil
Advancement: By character class
Kobolds speak Draconic with voices that sound like yapping dogs.
Combat
They begin a fight by slinging bullets, closing only when they can see that their foes have been weakened. Whenever they can, kobolds set up ambushes near trapped areas. They aim to drive enemies into the traps, where other kobolds wait to pour flaming oil over them, shoot them, or drop poisonous vermin onto them.
Light Sensitivity (Ex): Kobolds suffer a -1 penalty to attack rolls in bright sunlight or within the radius of a daylight spell.
Skills: Kobolds receive a +2 racial bonus to Craft (trapmaking), Profession (mining), and Search checks.
Kobold Characters
A kobold’s favored class is sorcerer. Kobold clerics can choose any two of the following domains: Evil, Law, Luck, and Trickery.
First of all, a fully-prepared large kobold tribe is going to have approximately 425 adults, with 25 of them 3rd to 8th level. Every one of them are going to have trapmaking skill, and using the ability to aid another in a skill use, the kobold lair will be brimming with traps of a high skill and high-CR nature. both nonmagical and magical. The leader is likely to be an 8th level sorcerer, and all of his lieutenants are likely to have at least some sorcererous ability - therefore, magical traps are just as likely as nonmagical ones. Lawful evil implies working together with precision and for the good of the community.
A tribe of kobolds would kiely only build passageways to accomodate them and their pets - only 3 feet high and maybe 2 feet wide? Also, they would use their dire badger pets to the utmost, especially to get flanking positions and sneak attacks.
Here's a really nasty thought - the passageways are only 3 feet tall, but the rooms are 6' high. Anyone who stands over 4 1/2 feet tall in a room triggers a trap in the room! The kobolds would have no problems bypassing these traps!
Without wasting a lot of space with repeat info, Check into the trap-making section of the DMG and Song and Silence. Spike pit traps, Crushing room traps, acid-floor traps, and Vaccuum Room traps are all quite nasty, and bound to wear on a party's resources. Since a one-time trap only costs some gold and 2XP, and a multi-use trap costs 20 xp, the kobold lieutenants and chieftain should have plenty of these set up to trigger around anyone over 2 1/2 feet tall.
One really nasty trap would be one that threw dimensional anchors at the party before it triggered another part that dumped them into another trap of some sort such as a collapsing room!
Kobolds can be quite nasty just looking at the existing core rules material, but you really have to work to make them a threat for really high-level PC's - just don't ever let them come into physical confrontation, or if you do, make sure they have enough minor magics to scare a party.
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Though it doesn't work in 3E as well as it did in 2E, writer Jean Rabe once said that she had a beautiful adventure setup in which a mage in a tower once died, and because of his fearsome power, no one bothered to invade his tower. It lay undisturbed for many years, until a tribe of kobolds and goblins moved in, braved the traps, and found that the mage had left his entire stockpile of magical devices and scrolls and potions behind. THe PC's went nuts when they kept encountering kobolds and goblins that were hopped up on potions of heroism, giant strength, fire breathing, etc. and who had access to necklaces of fireballs and the like. After a while, the PC's were even afraid of regular kobolds and goblins, because they couldn't tell if they had access to such magics.