Short one this time, which is far less than you deserve since my last post was about a month ago. That said, this series isn't done, just delayed. If we ever get together for a beer, I'll tell you all about it.
Continuing with major encounters, fortresses are an interesting case. They're not exactly settlements in the traditional sense, since their very nature is defined by some martial purpose. But they're hardly self-sufficient, so once all the monsters are cleared and the land secured, the fortress attracts settlers and, as a result, morphs from outpost to capitol.
In fantasy campaigns, fortresses seem to crop up with regularity—fortress populations have come up before. I base that on the prevalence of random tables used to describe who's in the castle and what the lord's minions are. Certainly, this has been covered in every pre-2nd Ed. version of D&D. The question is, why so many fortresses? If fortresses are smaller than settlements, why the need to give them so much attention?
My theory is that fortresses are adventure. They straddle the line between civilisation and wilderness. While they provide safety, they're erected in places of danger—they are islands of security in a sea of peril. Each fortress is a wildcard: what does it protect? will the characters be welcomed? are they better off within its walls or without? what's going on inside? 
Here's something you might use to populate fortresses in your campaign:
The Lord of each fortress may be described via the Random Noble Houses table. If that isn't enough to provide a hook, use the "What Goes On" section of the 69 Villages post.
Using the table above, here’s what I came up with for Minocra’s three major fortresses (my results are shown to show you what I was working with).
Hex #0812: Bala Keep (garrison: 15 dervishes) (What I rolled: keep; neutral noble; protecting a resource; dervishes & an elemental majordomo)
The knight-errant Bala completed his keep just over a year ago, and it's a beach-head for expansion into the interior. The keep is garrisoned by a troop of 15 dervishes—nomads who fought with Bala during his campaigns in the mainland desert—and the air elemental they've bound to their service. Bala's plan is to forge upriver to clear land and attract settlers, but he's beset upon by the native Usabir clans and their avian idols—it's a fair bet that he would welcome some extra muscle.
Hex #1005: Tower of the Brine (garrison: 9 men-at-arms) (What I rolled: tower; chaotic noble; protecting a village; men-at-arms & a dragon majordomo)
Through rooted in the seabed, only the uppermost storey of the Brine Tower is visible above the waves, though the sea crashes against the structure's white marble walls in a fury of thunderous churn and spraying foam. The tower was originally erected to serve the small fishing village to the west, but the village is no more and the place has since been occupied by the mad scion Shilaas and his deranged houseguard. Rumour states that Shilaas plunges the depths in search of favour from some powerful aquatic denizen—credence is lent to the stories via sporadic sightings of a massive sea serpent coursing about the tower's vicinity.
Hex #1518: Kubu Keep (garrison: 32 humanoids) (What I rolled: keep; chaotic mage; protecting a crossing; humanoids & a humanoid majordomo)
The sorcerer Kubu rules southern Usibir clans from his riverside keep. The mage's goblin raiders, led by hobgoblin "officers," sally forth regularly on raids throughout the surrounding jungle, occasionally taking Usibir captives, but most often scouring the riverbanks for the Frost Mirror, thought to be a portal that crosses to the Cold Realms. Kubu's purpose in the Cold Realms is unknown (if, indeed, that is even his goal), but he suffers no trespass.
Again, I think the random gods for guidance—this is another set of results that I would have struggled to come up with on my own.
As always, I'm interested in your suggestions and results—does this table work for you, and what's come of it in your campaign?
Remember your Mallory? Ever damn castle had some weird witch-curse-giant-damsel-prisoner-S&M dungeon thing going on.