The power and nature of the Girdle of Melian
 

How did it worked? I know Melian used it to protect Doriath. And, somehow, her apprentice Galadriel did the same to protect Laurelindorenan. But what was its effect on foes? Against whom was it raised? How long did it endure the attacks? Why it is not working anymore?

its not working anymore because beleriand sunk under the sea, which had the slight consequence of killing melian (it did didnt it?) it was used against orcs and all evil things to keep them from finding the path into doriath. but i think that if there were enough evil things they could penetrate it.

No, she left for Aman, to go into Lorien, and maybe serve Irmo again. Or cry. Or wait till Elwe is re-incarnated. Or write for The Wall Street Journal.

Tolkien fiddled around with Melian's actions after Elwe's death. In BoLT 2, she met and lived with Beren and Luthien, until passing to Aman, whilst in the more up-to-date verisons, of HoME 11, in which the Fall of Doriath is incomplete, there are several (sketchy) versions of it, but all culminate in her leaving for Aman.

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. Melian created it by using the power that she had.

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

Servants of Morgoth and unwelcome visitors.

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.

It would confuse and bewilder anyone who tried to enter without Melian's permission. It could still be broken, however, by something very powerful, as in Carcharoth.

If you remember the hobbits journey through the Old Forest, I believe it was similar to that. No matter how hard they tried, the winding paths led them to the Withywindle and Old Man Willow. Trying to enter Doriath, which was surrounded by forests, the paths would seem to shift and turn you round until they led you back out again. It always seemed like that to me.

Pardon my ignorance, but who or what was Carcharoth?

Wasnt it described as a shadowy mist that bewildered foes? I see it more like some kind of magical fog (mfog?) spiritual fog = spog?

drigel--Spog? Lol. I never heard that description. I'll admit I didn't do a lot of review except the "Beren and Luthien" chapter in SILMARILLION, and not even all of that. It said:

He was able to pass through because he was half-mad from grief for his father and his journey through the wilderness of Dungortheb
I don't off-hand know where to find any more detailed descriptions.