I'm still experimenting with this, and I believe there are many strategies - but here's what I know so far.
- Your Town Hall*. This is pretty straight forward, when keep trophies is your goal - losing it means automatic defeat. Consider upgraded walls around it.
- Your collectors and storage tanks. Attackers will target these, as it is one of the key objectives of attacking - to steal resources.
- Your defenses. Attackers will target these in order to stop losing troops, and they should be close to your Town Hall/collectors/storage anyway.
* Do note that if resource farming is your goal, you may be better served leaving your Town Hall exposed in order to solicit cheap shields, and distract focus from your storage tanks.
By exclusion, this means that your Barracks, Army Camp, Laboratory, Builder's Hut etc are not essential to be inside the wall. They still count towards your 50% building mark, which can result in a loss - so fit them when you can. They also serve as a good distraction for dumber units like barbarians, so keep that in mind. I have found that savvy players will place certain buildings like builders huts way away from the rest of their camp, to force attack units to hoof it. This can cost attackers valuable time, and may even save you from defeat when your defenses are exhausted.
Well.. rarely do you get a more definitive answer than from the developers themselves:
How does the Wall Breaker choose its target?
Steps 1, 2, 3
1. It will look for the nearest building/turret that is trapped (fully or partially) by walls
2. It will choose the shortest path towards that building and blow up walls that are in its way
3. After the wall is blown up, it will choose the next building that is still trapped by walls, run straight towards it, and blow up walls that are in its way
So Wall Breakers don’t target walls anymore?!?!!1!!!!?
No. Instead of targeting walls, they aim for the main prize, a building that is trapped by walls. The wall just happens to be in its way, and has to be blown up.
That building is the reason why you need that wall taken down, right?
Do Wall Breakers attack buildings?
No. They aim for the building and blow up the wall that is in their path.
Do Wall Breakers favorite junctions?
No. The Wall Breaker will run in a straight line towards the nearest building. When it encounters a wall, it blows it up.
Can I fool Wall Breakers by using spikes or short decoy wall sections?
No. They won't be fooled by those.
The two main takeaways from this that are interesting:
1. You can no longer lure Wall Breakers with "vestigial" walls.
2. Wall Breakers target the nearest enclosed building. Some have found that you can confuse them near the edge of your main village by enclosing something like a builder hut or tesla. ¡Viva the AI abuse!
Edit:
As added info, I've found this video from Flammy extremely informative. He shows the Wall Breaker aggro in action, and even has some crude measurements of aggro distance.
Best Answer
There is no official number, and user tests suggest it may be limitless; Hog Riders are just meant to jump walls, no matter how thick.
On supercell's user forums people like this guy have done user tests and complained that it appears the wall thickness doesn't matter, the hogs will jump it regardless. It is important to note this post was made in January 2014, Hog Riders could be patched at a later date.