The power and nature of the Girdle of Melian

[Salix]

The Girdle of Melian is no longeer working because (as I recall) at Thingol's death, Melian became distraught, and let her power lapse, in effect destroying the Girdle. That was my impression.

How did it worked?

Melian created it by using the power that she had.

But what was its effect on foes?

I believe that the foes just couldn't get through to Doriath.

Against whom was it raised?

Servants of Morgoth and unwelcome visitors.

How long did it endure the attacks?

Until Melian let her power lapse.

Please note that I haven't read the Silm recently, and I am working from memory. I am answering according to the impressions I had from reading the book.