The only acceptable type of trap in KAG involves crossdressing. Do not build traps. Listed below are reasons why you should not do this under any circumstance:
1. Waste of manpower.
2. Waste of stone and wood.
3. Lack of effectiveness.
4. The enemy can use your trap blocks as doors.
5. It can often weaken your defences.
Basically, making a trap is trading several minutes and a hundred or so stone for 1 kill.

Do not tunnel. The tunnel building is excellent and your team should be using it. The strategy of mining blocks below the surface to end up with a literal tunnel between two places, however, is terrible, and basically equivalent to griefing. Listed below are reasons why:
1. It is a massive investment of manpower.
2. Even if you build doors, the enemy can easily get in and use the tunnel against you.
3. It continues to waste manpower as the game progresses when fighting is going on in it.
4. Tunnels usually backfire.

Always stay on task. Manpower (time*people) is the most valuable resource in KAG.
If you have no idea what to do in the moment as builder, just go knight. More knights are always useful, no matter how incompetent. Below are basic guidelines.
Is the flag secure? If not, build a stone case around it.
Can knights get to the front line quickly? If not, build ladders or doors where they get stuck. Build a tunnel (the building) if the front line is far away. A tunnel should always be placed right next to the tent as well.
Do you have infrastructure? Here's what you should build, in order of importance:
1. basic defences (listed above)
2. knight shop
At this point, you should consider going out into the fray and building an advanced base or mining into the enemy base. KAG is not minecraft. You are a military engineer, not a civic engineer.
3. tree/wood farm
4. archer shop
5. A taller base: height is power
6. tunnel (the building)
7. siege workshop (and boat shop if it is a water map)
8. literally just go out on the front at this point.

Bringing materials to the front is broadly a bad idea. It's alright to bring your starting materials, plus some extra wood, but time spent gathering materials is time wasted. Materials will likely be on the battlefield, or an opposing builder with them will likely die while you are out there. It is perfectly okay if you die, just respawn and try again. It would be ideal if a knight is protecting you and your team has control of the front, but you still have a chance to run past them if they are distracted.
Doors are the most important thing. If you have spare wood or stone, just spam doors, as they almost never hurt your team (unless placed against an allied wall).
Try to box yourself in with a door exit, this offers protection and allows knights to assist you. The most effective place to box yourself in is against an enemy wall, particularly at its base. You should be careful when placing non-doors, as this can sometimes help the opposing team by reinforcing their wall.
At this point, you will want to dig into the opposing wall towards areas of interest or importance. Building doors is of the highest importance, especially when you finally bust to the other side of the wall.

If you don't know what to do at all, your game plan should be as follows:
At the start of the game, before the red barrier drops, just gather resources. Don't build anything (aside from maybe a flag cage).
When the barrier is about to drop (10-20 seconds left) switch to knight. Sit up against the red barrier and attack the middle when it drops. Even if you are a terrible knight, bodies help.
Observe builders. Learn by watching how they build. Learn by watching how they play. Take note of what works and what does not. This goes for allied and enemy builders alike.

I mentioned earlier that spikes are broadly worthless. They do have a niche, but expensive use. When placed on the front of a wall, they make scaling a wall significantly more difficult.

Broadly, you will want to build fortifications in the shape of a right triangle. This will allow allies to easily climb up them and enemies to be repelled more easily. This will also make retaking the fortification easier if it is taken by the enemy. It is also important to make many exits with doors into these buildings, plus a roof.