Khongorbayar,

A circle can be made by drawing two semi-circular walls.
1 - draw a wall from left to right, click only once, then
2 - draw a wall from right to left, double-click to end the wall.
3 - select the first wall, right-click and select Modify walls
4 - if necessary correct the exact X,Y coordinates, then
5 - change the Angle to 180 degrees
6 - click OK
7 - select the second wall, right-click and select Modify walls
8 - change the X,Y coordinates so that the end of the second wall is at EXACTLY the same position as the start of the first wall
9 - change the Angle to 180 degrees
10 - click OK

You should now have a circle made of two walls.

If you install the AdvancedEdit plug-in (PDF) you can use the function Connect walls to join the two sections of the wall.

Hans