Curves can be drawn by giving a second argument to rt and lt.
The optional second argument for rt and lt is a turning radius.
The turn is made while tracing out part of a circle of that radius.
rt 180, 50
If you provide a radius of 0, the turtle will just pivot in place as with a normal turn.
A complete circle can be drawn by turning 360 degrees (or more).
lt 360, 25
To draw a circle, the turtle must start on the edge of the circle. For example, if we wish to draw a circle around a dot, we must move the circle to the edge of the dot to begin:
dot yellow, 100 fd 50 rt 90 pen violet, 3 rt 360, 50