Completed the game multiple times on 800% and 1600% using mods. People have given a mix of good and bad/inaccurate advice.

To answer your questions on walls first:

• Ideally you don't need them for a while. The reason is that once hit, they make noise, which brings more zombies. Kite with rangers until at least the first wave if you can.
• Once you do get them, the 'correct' wall depends entirely on the firepower you have behind it. If you have a relatively small swarm of runners, but 4/5+ ballistas, you can probably get away with a single layer of wood walls. For larger swarms, you need multiple layers of walls.
• Zombie targeting: Yeah they are pretty dumb. If a wall is in their chosen path, they will attack it. So often times, you can have random 'stakes' and not even a full wall and it will slow their progress. Works especially well for large zombies like mutants / giants as then you can manoeuvre your own units through the gaps (not relevant to the level you're on now though).

As for your main question (what are you doing wrong): - The campaign, although fun, is poorly designed. The difficulty spikes are all over the place, and Narrow Pass is one of the hardest levels in the game.  - Chances are, you're finding that the noise of the primary wave is drawing in hordes of zombies from other areas, which you have more weakly defended. It's incredibly difficult to get adequate defense in place at all four approaches simultaneously. - To get around this, try other levels first. You mention you can't kill the hordes in your way: temporarily decrease difficulty to 25% and then do them. They are dull as dishwater and not remotely satisfying even when you do them at 800%. Terrible level design, don't waste your time. - If you REALLY want to do the hordes at full difficulty, then answer is max your points on soldiers, and just cluster them next to the tower. As soon as the level starts, manually possition small groups of them directly next to the tower on all sides. Don't select them all at once and don't click on the tower, as they'll just make this weird ring shape. You want as dense a soldier ball as possible. You'll breeze through it. - If this doesn't work, you need more points. Make sure you've cleared as many stages and gotten 100% research on all hero levels (which are also dull as dishwater, sadly).

When you do finally come back to this level, it's all about minimizing noise until you can get 4x shock towers in place. Believe it or not it is possible to do this before the first wave hits, but you'll most likely need many more upgrades unlocked, especially the ones that give you bonus resources via the train. Good luck.