I just assumed she kept humans out of Narnia from Archenland and Calormen. The Pevensies came from the North. Maybe she didn't expect that. At least thats what I came up with for myself.

As for the Lone Island: Do we know that they were inhabited during the Hundred Years? I can't remember if that was mentioned in the books or not. If not, then maybe the humans on the Islands were killed of when the Witch took over (along with any humans in Narnia at the time from King Frank's line) and were only reinhabited after the WW was killed.

I know Lewis didn't plan out the Chronicles and thus this might just be an error on his part. However I like to think that maybe he did realize this but wanted readers to use their imagination to fill in the gaps.
Hope this helps!

It could be perhaps that the White Witch was looking for people not native to the Narnian world. The Calormenes were dark-faced and easily identified and I doubt any of the Lone Islanders would want to visit Narnia when it was reigned by the White Witch. But Edmund did mention in the Voyage of the Dawn Treader that the Lone Islands were "Narnian before our days, in the days of the White Witch". The White Witch could tell that Edmund was not from there since he didn't know who she was or anything else about Narnia.

The Witch's winter very likely made travelling through the mountains from Archenland close to impossible. Heavy snows in the mountains would have shut off all travel from Archenland (as well as from the Western Mountains). If her winter was severe enough, then the oceans could have frozen, forbidding the Lone Islanders from reaching Narnia by ship. As was pointed out by Valiant, an invasion from the North was unlikely, particularly as any people would have to get past the giants (which whom she was likely in league), and would have to cross the snow covered moorlands.

^Actually that's a question I've always had DiGoRyKiRkE. Was the winter just in Narnia- the country, or in the entire world? I had always assummed it was just the country and that all humans were to terrified to enter.

^I agree. It would make a lot more sense if only Narnia the country was under the winter enchantment. Although, that does make one wonder, why didn't Jadis expand her control and take Archenland and Calormen? Was she not powerful enough?

I agree that the Witch's magic kept everyone from traveling to or from Narnia, and I also agree that it was just Narnia the country which suffered from the 100 year winter. In The Horse & His Boy the Calormenes knew that Narnia had been under a winter curse, but they say nothing about Calormen being affected.

I think Jadis was not yet powerful enough to take over the entire world, but I definitely think it was on her To Do list. Being immortal, she had time to wait and gain enough magical and military strength to take Archenland et cetera.

It took political strength (being Narnia's legal executioner), magical strength (the hundred year winter), military strength (her army of giants and evil creatures), and the false pretense of heredity (being a daughter of Eve) in order to gain control of Narnia. But she could only take Archenland and Calormen by shear power of force, since she had no political advantage there. I think she automatically became empress of the lone islands when she crowned herself queen of Narnia. I doubt she ever went to the lone islands personally, much less conquered them. The lone islands were Narnian since before the witch.

This does leave me with the question: If Jadis' winter stopped all travel to and from Narnia, how is it that Bree and Hwin were able to leave at such a young age?

This does leave me with the question: If Jadis' winter stopped all travel to and from Narnia, how is it that Bree and Hwin were able to leave at such a young age?

Do we know how old Bree and Hwin are? It is possible that they were captured after the winter had happened. (Or if they are extremely old before the winter happened. ) I don't know about Narnian animals but most animals in our world reach maturity quickly. Thus in the years between the end of the winter and HHB they could be captured and have become adults.

I wonder where they managed to find food for themselves during the winter. If the boarders were all closed how did they get foods like potatoes?

Yes, the Witch's winter only affected Narnia. It is likely that either the Witch killed what few humans there were or they fled to Archenland or one of the islands. She was surprised to see Edmund because there hadn't been any humans seen for roughly a hundred years. After a hundred years even a witch's memory may be a little fuzzy.

The Lone Islands were inhabited during this time period, but I highly doubt the Witch ever went there. He [Jewel] told how King Gale, who was ninth in descent from Frank the first of all Kings, had sailed far away into the Eastern seas and delivered the Lone Islanders from a dragon and how, in return, they had given him the Lone Islands to be part of the royal lands of Narnia forever. LB, What News the Eagle Brought
Based on this quote, the title Emperor/Empress of the Lone Islands was a hereditary title passed down from King Gale and the Witch took it as one of her titles since it went with the kingship/queenship of Narnia. Not that she could rightfully be Queen, but since she claimed the title of Queen she might as well have the rest.

If Jadis' winter stopped all travel to and from Narnia, how is it that Bree and Hwin were able to leave at such a young age?

I always thought they left Narnia after the winter. The Pevensies are good deal older in HHB than they are in LWW so it must be later in their reign.

I wonder where they managed to find food for themselves during the winter. If the boarders were all closed how did they get foods like potatoes?

A black market? I've actually thought about this quite a bit and some kind of black market is the only thing I've been able to come up with that makes even the tiniest bit of sense. The pass to Archenland probably had snow in it most of the time, but even during the Witch's winter it thawed occasionally based on this quote: "This is no thaw," said the dwarf, suddenly stopping. "This is Spring. What are we to do? Your winter has been destroyed, I tell you! This is Aslan's doing." LWW, Aslan is nearer And even when there wasn't a thaw some animals that were more suited to the winter climate might have been able to get through.

A black market? I've actually thought about this quite a bit and some kind of black market is the only thing I've been able to come up with that makes even the tiniest bit of sense.

I think I read an interview where Doug Greshem suggested the same thing. The only other I could come up with was green houses. It never occurred to me that some Narnians might go and get the food. That makes a bit more sense than whomever they are trading with coming in.

In the Horse and His boy, which takes place only 14 years after LWW, there are clear references to adult Narnians- presumably inhabitants of the various islands belonging to Narnia or refugees who fled to Calormen but retained their identity as Narnians, and returned after the overthrow of the Witch.

As for the Lone Island: Do we know that they were inhabited during the Hundred Years? I can't remember if that was mentioned in the books or not.

They were, I believe. In LB, Tirian tells Eustace and Jill of King Gale, ninth after King Frank, who sailed East and delivered the Lone Islanders from a dragon and Narnia has owned them since.

In PC, when the Pevensies are wandering around in the Cair Paravel treasure chambers, Edmund mentions a suit of armor that Peter wore during a tournament in the Lone Islands.

Given that, I would guess that they were inhabited during the Hundred Years of Winter. It would be strange if they were uninhabited throughout those Hundred Years and then just a few years later, somehow there's a big enough population to have a tournament which the Pevensies attend.