My answer in this question What is the minimum radius of a circular corridor for the walls to appear straight?1 discussed various architectural methods to trick someone into thinking that a circular corridor was actually straight. Most of those methods that might work indoors wouldn't work so well for an outdoor wall, but possibly something similar might work to some degree out side.
What if the wall is not perfectly circular?
Perhaps the wall has semicircular towers no higher than the wall itself attached to it at regular intervals. So when the viewer of the wall looks to right or left the sections of straight wall between towers will look shorter and shorter with increasing distance until the viewer will only see the front edges of the semicircular towers one behind the other in the distance. And the farther away each tower is from the one nearer to the viewer, the less the front of it will seem to project beyond the front of the tower near to the viewer. Finally the farthest off towers will be impossible to tell one from the other.
Or perhaps the wall will have independent bastions in front of it at regular intervals, connected to the main wall by bridges or walls. So as a traveler gets closer and closer to the wall and is closer to being able to sight along the length of the wall to see if it is straight or curved, the traveler will have to look through any openings that might be in the bridges or wall connecting the bastions to the main wall.
If there are no openings in walls connecting the bastions to the main wall, each space between two bastions will be a separate bay of the wall structure. Once a traveler is closer than the outer edge of the bastions he will be able to see only a short stretch of the main wall.
Possibly a traveler reaching the city or country enclosed by the wall will travel across a plain and the wall will slowly appear as a dark line on the horizon and appear taller and taller as the traveler approaches.
The wall might have a moat many miles wide in front so the traveler will have to approach along a causeway or bridge for miles, with the nearest other causeway or bridge miles away over the horizon to the right or the left. Thus the traveler will be unable to walk around the outside of the wall and notice from the sun in the day and the stars at night that the wall is curved.
If your story is a fantasy you can set it on a flat world that could be as large as needed and your wall can be as many thousands or millions of miles or kilometers in diameter as it needs to be to be circular and appear straight. And in a science fiction setting you might be able to create an analog of a flat Earth in some gigantic artificial construct in space built by a highly advanced society. Something like some of the constructions discussed in Larry Niven's "Bigger Than Worlds": http://www.isfdb.org/cgi-bin/title.cgi?133302