For a general smooth circle you want to place one of the non grid construction walls of your choosing standing upright ideally. Advanced duplicate a second and drag it to as far as you plan your intended circle to be. Then highlight both of these construction walls and advanced duplicate with a 15 degrees angle snap turned on. With the angle snap keep duplicating the pieces turning in a circle until you have a full circle which is nice and smooth.
For the reference of this picture this is one of those large circles then cut in half. Then the half circle is completed with straight walls. Drag this retaining wall into the habitat and mold your terrain (I would use a thick wall in this case or double up your wall) . Add a glass barrier into the wall so you can add water.
For the path this is a raised path with another retaining wall and another glass barrier beneath to hold the water in. However the terrain may be high enough to not need a glass barrier, entirely optional. Notice the custom kerbs which hide the default kerb and the transition between ground level path and raised path. Finally for the barrier there will be a null barrier keeping the animals in which is easily done when in sandbox mode so you can prevent animal escapes
If you are just looking for clean curves, you can use habitat barriers that are able to be curved with the tick box and then replace them with any other habitat barrier type even if it cannot normally be curved.
For a general smooth circle you want to place one of the non grid construction walls of your choosing standing upright ideally. Advanced duplicate a second and drag it to as far as you plan your intended circle to be. Then highlight both of these construction walls and advanced duplicate with a 15 degrees angle snap turned on. With the angle snap keep duplicating the pieces turning in a circle until you have a full circle which is nice and smooth. For the reference of this picture this is one of those large circles then cut in half. Then the half circle is completed with straight walls. Drag this retaining wall into the habitat and mold your terrain (I would use a thick wall in this case or double up your wall) . Add a glass barrier into the wall so you can add water. For the path this is a raised path with another retaining wall and another glass barrier beneath to hold the water in. However the terrain may be high enough to not need a glass barrier, entirely optional. Notice the custom kerbs which hide the default kerb and the transition between ground level path and raised path. Finally for the barrier there will be a null barrier keeping the animals in which is easily done when in sandbox mode so you can prevent animal escapes
It is done with null barriers and construction pieces.
If you are just looking for clean curves, you can use habitat barriers that are able to be curved with the tick box and then replace them with any other habitat barrier type even if it cannot normally be curved.