Although the effects of the Qhapaq Ñan and a typical Mountain Tunnel are identical (they both allow rapid movement through mountain ranges), the Qhapaq Ñan may have some hidden superior traits to Mountain Tunnels. First, this improvement is available in the Ancient Era, while its counterpart becomes available in the Modern Era. This makes Trade Routes sent to and from the Inca much more fluid and effective, and also facilitates exploration since they cannot be blocked off by a long mountain range. Remember that Qhapaq Ñan and Mountain Tunnels can be built on Mountains in neutral territory, but since the Qhapaq Ñan is available much earlier, as the Inca, you can utilize this to declare a surprise attack on civilizations who mistakenly believe they are safe because they are protected by mountains. Even when Chemistry is unlocked, there are not many reasons to build Mountain Tunnels over Qhapaq Ñan, since the Qhapaq Ñan is built by Builders rather than the more expensive Military Engineers. Since some other civilizations do not have incentives to build Encampments but have to do so to gain access to Military Engineers, the Inca do not have to waste a district slot if they do not wish to pursue conquest; therefore, the Qhapaq Ñan still has the upper hand even when the Modern Era comes.
Bear in mind that your opponents can use Qhapaq Ñan/Mountain Tunnels built by you, which means that they can use your bonuses against you if you go overboard with this improvement and put it everywhere. When wars are declared on you, immediately place your military units to block entrances and exits and that lead to your core territory; otherwise your cities may fall in one fell swoop.
The Qhapac Ñan, also known as the Andean Road System, is a network of roads used by the Incan empire. What makes this site so remarkable isn't simply the distance covered – over 30,000 km – but also the terrain through which the roads travel - the mountains of the Andes, deserts, and lush rainforests. While the roads were used for transporting goods – usually by llama or alpaca – there was also in place a speedy messenger system. The chasquis were quick and agile men who would run through the extensive network of roads to pass messages to one another at relay stations until the message reached its final destination.
The Andean Road System was a truly innovative means for keeping such a vast empire connected, and to some extent, controlled by a central power.
Move a military unit into an enemy's owned tile using a Mountain Tunnel (or Qhapaq Nan)