City is involved in a very long war along a front which is subject to some kind of warping, such that you can never actually get there—just fire through it. At the end, protagonists suggests that the front is a mirror, and the city is battling itself.
This sounds like David Masson's Traveller's Rest (which has been the subject of several other story-identification questions here: first Short story; war where speed of time changes with distance from front line and later A war on a planet where time passes slowly at the poles and What story is set on planet where time runs faster as one approaches the pole?).
The story itself is available in full here. The final few paragraphs are as follows (emphasis mine):
“XN 2. Things are livelier than ever. They certainly are hot stuff. Every new offensive from here is pitched back at us in the same style within minutes, I notice. That new cannon had only just started up when back came the same shells—I never knew They had them. Tit for tat.”
Into H’s brain, seemingly clarified by hunger and exhaustion and much emotion, flashed an unspeakable suspicion, one that he could never prove or disprove, having too little knowledge and experience, too little overall view. No one had ever seen the Enemy. No one knew how or when the War had begun. Information and communication were paralysingly difficult up here. No one knew what really happened to Time as one came close to the Frontier, or beyond it. Could it be that the conceleration there became infinite and that there was nothing beyond the Frontier? Could all the supposed missiles of the Enemy be their own, somehow returning? Perhaps the war had started with a peasant explorer lightheartedly flinging a stone northwards, which returned and struck him? Perhaps there was, then, no Enemy?
“XN 3. Couldn’t that gun’s own shells be reflected back from the Frontier, then?”
“XN 2. Impossible. Now you are to try to reach that forward missile post by the surface—our tunnel is destroyed—at 15º 40’ East—you can just see the hump near the edge of the I/R viewer’s limit—with this message; and tell him verbally to treble output.”
The ragged hole was too small. H left by the forward port. He ran, on his “walker,” into a ribbon of landscape which became a thicket of fire, a porcupine of fire, a Nessus-shirt to the Earth, as in a dream. Into an unbelievable supercrescendo of sound, light, heat, pressure, and impacts he ran, on and on up the now almost invisible slope . . .