Gammelstad means simply "Old Town" in Swedish, and in the context of Luleå refers to a location about 10 km away from the modern Luleå. It is not an "old town" in the sense you would see some cool medieval stone buildings; rather, it's a church town (kyrkstad), which is quite a particular kind of place. Church towns of this kind mostly existed only in Norrland in Sweden, although some were also known in Ostrobothnia, Swedish-speaking part of Finland on its west coast, and in Sami areas in Finnish Lapland and North Norway.
Church towns consisted of small and quite basic cottages, clumped around, well, a church. They were meant to house churchgoers who lived too far from the church, and could not attend a service and return back home on the same day. This explains why they were only necessary in remote and thinly populated (at the time) areas, such as Luleå. Cottages were mostly privately owned, so in other words everyone had their own cottage. (Attending church was kind of a big deal, yes.) A particular subtype of church town was a Sami church town (lappstad, Sami town); the semi-nomadic way of life of Sami people naturally meant it was difficult for them to attend churches. The Gammelstad of Luleå however is not a lappstad; Luleå has always been a Swedish, not a Sami settlement. The best surviving lappstad is located in Arvidsjaur in Ume Lapland, farther south. It's still however an order of magnitude smaller than Gammelstad.
Most church towns didn't survive to this day for a few obvious reasons. People don't really saw any obvious need to preserve a bunch of rather ordinary shacks, and they were usually torn down and replaced with new buildings, when these church towns became no longer necessary. And of course "a bunch of closely standing wooden cottages" is basically a synonim for "a huge fire just waiting to happen"; many of these church towns burned down.
Overall, 16 church towns in Sweden have at least a few buildings currently remaining of them, but the Gammelstad of Luleå is by far the biggest one (which is why it was declared a UNESCO World Heritage site). It got lucky for a few reasons. The settlement in this location predated the actual city charter of Luleå; some sort of a marketplace existed here already in the 12th century, and the current church was built in 1492 (before that there had been an older church here as well, but we don't really know much about it). But one of the things you need to know about the settlements on the coast of the Bay of Bothnia is that they often had to be moved over time, because the whole area experiences rapid postglacial rebound; the ground, formerly pushed down by the glacier in the Ice Age, now pushes back still relatively rapidly, and the sea level falls down a few millimeters every year. Thus Gammelstad was originally a fine location for a marketplace, thanks to having a nice harbor, but eventually the harbor became too shallow. This became painfully obvious when Luleå actually became a city in 1621, and the city was moved to its current location in 1649. The church town in the old location was thus saved from any possible redevelopment. Its luck continued on, in that it never experienced any major fires, or war destruction. (In particular in 1721 the Russian fleet pillaged and burned down most Westrobothnian towns, up to Piteå, and was moving up to Luleå just when a peace was signed.) It continued to be the actual government seat and parish center until the 19th century, and thus Gammelstad had been very actively used especially until that time.
Currently Gammelstad consists of 424 (!) cottages with 555 rooms, all clumped around the old church of Luleå (also known as the Nederluleå (Lower Luleå) Church). They all have very similar design, and form quite an unusual environment. Almost all of them are still privately owned to this day, and used as holiday residences; although they now have electricity, there is generally no running water. There is also an open-air museum in the area, named Hägnan. Gammelstad is certainly worth visiting; I actually visited it before Luleå proper, and, as I hadn't read anything about it before, was quite impressed; I discovered something genuinely new for me, as I had no idea about these church towns at the time. It can be reached from Luleå by a regular city bus, although of course a car is more convenient. The Ore Railroad also passes right by, but I believe trains don't stop here anymore.
1. If you're coming to Gammelstad from the direction of Luleå, this sign will remind you that you're entering a UNESCO World Heritage area :)
2. I was coming from the opposite direction, and the first thing I saw was this large but still nearly overflowing parking lot. Huh, is the place thatpopular? (Spoiler alert: generally no, there was some event at Hägnan open-air museum.)
3. And we're entering the churchgoer cottage area!
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5. They really are quite fascinating; very similar to each other at first glance, yet no two are completely identical. The uniform red paint appeared some time in the 19th century; before that, they were unpainted except for windows.
6. In the center of the area is the old Nederluleå church. It is the biggest medieval (1492, as I mentioned) church of Norrland. As usual, the current belltower is much newer, dating from 1851.
7. Accidentally caught a Luleå city bus here finally. Nice color.
8. I didn't check out the visitor center actross the church, but I read that you can get a tour of the church there.
9. Interesting cover, in the shape of an old 5 öre coin. I believe öre (1/100 krone) coins are no longer in use in Sweden.
10. That's one good looking trash bin.
11. Another alley of Gammelstad...
12. ...that leads to the Hägnan open-air museum. Here I learned the reason why there were so many people; there was some sort of a trade fair here, mostly about restoration of old interiors and items. At least on this day the museum had free entrance.
13. There are a few animals on the museum grounds, but no rabbits (l love rabbits).
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15. Window frames restoration, I believe.
16. Open-air museums, showing mostly old rural buildings and way of life, are relatively common in the Nordics (the most famous of course being Skansen in Stockholm and Seurasaari in Helsinki, but there were two such museums even near the fairly modest-sized city of Vaasa, where I lived in Finland). I've seen quite a few and wasn't that interested in the museum as such.
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20. Back to Gammelstad. This is the local school.
21. Normal residential street. Church cottages take up most but not all of the Gammelstad area; there are more normal and modern detached houses as well.
22. Small cafe. If you look at that menu they have outside, you'll see they have a lot of dishes with reindeer (ren in Swedish). These are, of course, northern lands and reindeer dishes are appropriate, even though Luleå and Gammelstad are actually still pretty far from the reindeer husbandry areas.
23. Some other tourist taking pictures at the bus stop. You can see here there are some two-storey cottages as well, but they're definitely a minority.
24. There's quite a bit of information about Gammelstad online (funnily enough, Wikipedia articles in English and Swedish are quite brief, but for some reason the Finnish one is much longer and more detailed), but I couldn't really find the answer for a very obvious question, namely, how old these cottages actually are? The closest thing to an answer that I found was that on the first actual map of the area, made in 1817, many of the existing ones were already present apparently more or less in the same shape.
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27. Not sure what this sign or monument stands for. The reindeer in the coat of arms looks roughly similar to the one on the coat of arms of the Västerbotten Province, but stars on that one are spread evenly around the field instead of just forming a Big Dipper shape on top, like here.
28. There's also some more modern monument nearby in this small park next to a church.
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30. And a local kitty! Kitty says hello and we now depart the coastal Norrbotten lands down the Road 95, along the Lule valley and towards the Scandinavian mountains range and reindeer country. And if you want to have some more views of Gammelstad (in winter), there's a website with air panoramas: gammelstad360.se.