"The Narrow Pass is a key strategic location that the Empire wants to control at all cost, as it is the only way to cross the Blood Mountains. The narrow canyon is divided by the Wolf River and is only connected by a narrow strip of land, providing passage through the middle of the valley. Although it is a narrow and complicated place to build a colony, it is of vital importance to the Empire. There are many infected at the bottom of the canyon who ended up there long ago. Move forward little by little to make your way through the canyon and kill all the infected and the approaching hordes."
Narrow Pass may be your first major stumbling block, as it is far less forgiving than any map before it. The shape of the canyon massively constrains your building area and you have to fortify against four different avenues of attack almost immediately.
To start, focus on clearing out the center. Each riverbank has a distinct chokepoint not too far from the Command Center which you can work towards -- the southwest bank has two, even. Clear out the way to the wood and stone supplies and get started. Be careful with your Fisherman's Hut placements along the river to make sure you're getting the most out of them -- in this claustrophobic space, you'll need all you can get. Once you've secured the chokepoints, build up a small army of your choice -- a "twelve-pack" of Soldiers is a good option if you can push up to the iron on the Northeast bank.
After getting said army, start clearing out the riverbanks, one at a time. Pushing up the Northwest bank first is ideal, since it has a gold deposit that will be highly useful. Be sure you have your defenses up and staffed before you start clearing -- working up one bank will cause enough noise to attract the Infected on the opposite bank, who will run themselves en masse through the heart of your colony to try and reach your clearing party if you don't have something in their way. In addition, Swarms can appear on either bank, and the Swarm alert isn't precise enough to tell you which bank they're on, so make sure all four approaches are fortified.
Once you've cleared one bank, clear the opposite one next, moving your defenses up as far as possible to squeeze as much room out of the canyon floor as you can. After clearing out one route, repeat the process in the other direction, expanding as you go. Make sure your defenses are comprehensive, as well, as the final wave you'll have to face will send swarms along ALL four banks, making using mobile reserves to hold them off all but impossible.
Narrow Canyon also changes dramatically based on what technology you have unlocked. Having upgrades to the Fisherman's Cottage, like the very early Ropes and Pulleys, helps immensely. Having the Cottage upgrade is almost mandatory, as trying to squeeze 1000 people onto the canyon floor in tents may be outright impossible. If you've gotten all the way to Stone Houses, it becomes dramatically easier, though this then offsets housing requirements with needing more room for Mills. Think hard about the placement of your Wood Workshop and Stone Workshop (if you have it) -- you can't move them once you place them and space is at a serious premium down here. Be careful about what you're building on, as well -- you only have so much farmland and forest available, and you'll want to make use of all of it.