Just quoting Matt Galas a bit for those who didn't bother with reading the thread linked. It is interesting stuff, really:
http://www.swordforum.com/forums/showthread.php?80863-Belgian-Longsword-Rules
"Here's a simplified version of the rules used in Belgian fencing guilds during their yearly competitions with the longsword. The rules are the "common denominator" between the various rule sets; individual guilds had some variations on these. These rules were in effect in Northern France (Paris, Lille) and Belgium (Ghent, Bruges, Brussels, Antwerp) from at least the 1540s until the late 1700s.
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What type of longswords were used in these bouts? (foils? sharpness?)"
All the evidence points to the same type of longsword foils that the Germans used (what people are normally referring to as a "Feder" in our community). They were almost certainly blunt. The rules generally say that you have to strike with the _flat_ not the edge.
"What type of clothes/PPE were worn?"
The rules appear designed to replicate combat in street clothes, but the rules make references to "wambaes" (gambeson), "cassacke", and "accoutrement", so it is likely that some kind of padded garment was worn. Likewise, there are references to some kind of head-gear: "slagsweerdevryhoete" (literally, "battle sword free hat"), "chapeau" (hat), and "bonnet" (hat, bonnet). Gloves were mandatory, and there were fines for fencing without gloves.
"What level of intensity were put into blows, and what constituted an acceptable hit?"
Hard to say, except that there were fines for injuring your opponent. Blows were with the flat or the point. An acceptable hit was above the belt, and above the elbows. It appears likely that the face was taken out of the thrusting zone (this appears in rapier rules), but it's not explicitly documented in the longsword rules. In terms of intensity, put it this way: The Guild of St. Michael in Lille employed a surgeon to tend to the fencers injured in the hall.
"How competitive would of the bouts been?"
Extremely. There are lots of references to arguments and disputes. Many rule-sets explicitly say they have been re-drafted because there were so many disputes. (Nothing changes.) Some of the target and technique limitations must be seen in this light; they were clearly afraid of this degenerating into a brawl. (This can be documented _many_ times in Germany, with some city councils outlawing competitions for this reason.)
"What type of arena were the bouts conducted in? (outdoors? within barriers?)"
Usually outdoors, in one of the town squares. There are references to benches, which probably helped define the fighting area.
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"Do we have any other similar sets of rules from elsewhere in Europe?"
Yes, the German rules are very similar, although they don't explicitly limit the target area. German rules contain the same rule about how the higher hit wins, so the target tended to be the head. Joachim Meyer confirms this, when he says that most of the fencing in his day aimed at the head. German rules tended to explicitly limit grappling, pommel strikes, and often thrusting (at least with the longsword). Thus, in Germany you end up with a set of rules that strikes largely at the head, using cuts only.
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It's important to view these rules in context: They were intended for public competitions, for the very prestigious title of "King". There was a _lot_ of money put into these events. The figures are pretty impressive in terms of costs of wine & food, not to mention prizes. Many of the rule sets contain mentions of heated disputes over the rules, indicating that these competitions were taken very seriously. There were also rules establishing fines for injuring an opponent.
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To give you an idea of how many people were taking part in these competitions, here is some information I transcribed tonight from the archives of the Guild of St. Michael in Bruges.
In 1699, the guild played three prizes:
1) For the prize with the longsword (slaghsweert), 50 guild brothers participated. The prize was won by Theodor Haeghebaert.
2) For the prize with the rapier & dagger (rapier end poignaert), 61 guild brothers took part. The prize was won by Niclaijs de Smidt.
3) For the prize with the single rapier (deghen alleen), 64 guild brothers took part. The prize was won by Ignatius de Vogelaere.
Bear in mind, this was an internal competition of a single city's fencing guild. Oh, for the days!"
I have quite a lot of images depicting these types of events. Some can be seen here:
http://www.hroarr.com/articles-reviews/galleries/fechtschulen/