In the world of fantasy, there are many places where one reality overlaps with another. Often times, crossing through these places comes as an unexpected shock - you're in one place one minute, a few steps later, everything is (perhaps radically) different.
But what of those points of intersection that are perceived and located? Having control over such a resource would provide a significant tactical advantage against potential invaders as well as a healthy profit for adventurous merchants seeking to peddle their wares in an exotic locale.
While gates such as this might come in a variety of shapes and sizes, the gate presented here was surrounded by heavy iron doors set into a stone wall. A three-story tower and small barracks sit on either side of the wall, ready to repel those who would try and force their way through the gate as well as stop those who might try to sneak across from the other side. The structure as a whole is not exceedingly tall nor is it very wide. Were it to exist in a forest, it could easily be bypassed by without ever being seen.
The paths that traverse realities can be fickle, unreliable things. Some are consistently active while others are only usable at certain times of the day or the lunar cycle or some other suitably obscure period of time. Other gates can only be opened with some kind of key that reacts as it passes through the portal.
For example: The Stonewall Gate - as we shall refer to it - is active every full moon for a period of 12 hours. It then closes until the next full moon. The iron doors of the gate can be locked, opened and closed at any point. The doors exist and operate only in the reality in which they were built - if a second set of doors cover the gate in the connected plane, travelers will not be able to pass through without some means of forcing them open.
When a creature passes through the gate, they come out in the new plane on the side opposite of the one they entered. This means that the gate can be used simultaneously from both planes. It also means that when a creature enters on one side, a second creature could return on the opposite side without the first creature's knowledge - hence the importance of the fortifications.
Two applications for the fortified planar gate immediately spring to mind:
First, both sides of the gate are located in a large, busy cities, the gate is important for allowing trade between two peaceful (or at least somewhat agreeable) civilizations in neighboring planes. Such a gate would probably be active on a daily or weekly basis and the cities on both sides would go to great lengths to protect it. In the event of a siege, the gate could be used to bring in supplies; should the city be overrun, it would become an escape route.
Second, an old forgotten gate exists in obscure corner of large forest. The gate is only ever active once in a long time and has fallen into ruin and disrepair. Any civilized races who happen upon it (and are not well versed in arcane history) might think it an eccentric piece of architecture by some rich noble with more money than sense. Less savory races would be happy to call such a place home, despite it being little more than a wall that doesn't block anything housing a door that doesn't go anywhere.