arched down to either side

[suprunp]

As Eragon formed the last word with his thoughts, a thin, dark line appeared at the base of the spire. It ran upward fifty feet and then split in two and arched down to either side, tracing the outline of two broad doors.
(C. Paolini; Inheritance)

What role does the 'down' play here?

Thanks.

[Rover_KE]

It tells you that the two lines fell back to the ground.

Rover