Default arena is rectangular, but you can draw own shape of the arena.

First, hide the rectangular arena by going to the Display Options (default key shortcut Shift-P) and checking Hide arena boundaries box.

Arena without the boundaries will look like in the picture below, only the rulers with dimensions will be visible. You can hide them too choosing Hide arena ruler in the Display Options.

You can also hide the arena boundries later and use the rectangle as a guide when drawing the shape.

Now choose the Curve tool from the toolbox and draw the new shape.

You can use the Curve tool options to turn the lines into curves, change its width, colour and other properties. By double-clicking on the lines you can add more points to the shape.

Finally, lock the new shape, so you don’t accidentally move it when working with obstacles.