The Gates of Haven

Haven is one of the few walled cities in the Upper Realm, partly because it is very urban, but mostly as a tactic to keep magic out. The double ringed walls are several meters high and made of chert stones and mortar. There are gates at each of the cardinal directions made from Stature Oak, a rare tree characterized by resistance to magic. Stature Oaks are only found in the newest parts of the Upper Realm, said to have been planted by hand by residents of the Lower Realms. (Scientists theorize that because the realm was created by the magical will of the immortals of the Lower Realms, the entire land of the Upper Realm is infused with magic at its core, but Stature trees were not born at the creation of the Upper Realm, but rather carried by hand up from The Lower Realms and planted, thus they do not carry the innate magic-ness of native Upper Realm plants.)
The gates are not simply a door, as one might assume. While yes, either ring of the gate does have a pair of double doors that open to allow travel in and out of the city at the judgment of its Heroes, that alone is not enough to keep magic from entering this city. Between the two rings of wall the city grows its own Stature trees (grown from cuttings of known Stature trees found elsewhere) that provide an extra layer of magical protection from outside attacks, but also supply a range of Stature trees (oak is by far the most common, but there are aspens and pines in high quantity too).
However, these trees also host nora-qwerls (a type of small rodent, much like a squirrel but with golden stripes across brown fur, that are known to love the Stature trees. They flock to these trees in nature and live in them almost exclusively.) These nora-qwerls are naturally inclined to seek out non-magical substances (and have been used to locate most known wild Stature trees), but have been trained to react to magically potent things by screeching. All those who visit the city of Haven must pass through this ring of Stature trees and the nora-qwerls. If the alarm sounds, they are taken away for further investigation, but if the nora-qwerls pay them no notice or especially if they swarm (as they do to people with little to no magical blood), then visitors are swiftly allowed into the city.