Star Fortress is a level 65 flashpoint accessed from Odessen that requires you to have made it up to chapter nine of Knights of the Fallen Empire. (In case that doesn't already give it away, it was added to the game with the release of KotFE in October 2015.) You cannot enter a Star Fortress any other way, even if you are grouped with someone else who has them unlocked. They also aren't part of the group finder and need to be entered straight from the Alliance base.
While every Star Fortress is the same in terms of gameplay, there are technically supposed to be six different ones: one each above Alderaan, Belsavis, Nar Shaddaa, Hoth, Tatooine and Voss.
Unlike regular flashpoints, the Star Fortresses are not designed for a group of four players. Instead they have an easy solo version and a heroic mode, with the heroic mode still being soloable if you are both sufficiently geared and skilled. The heroic mode follows the same basic format as the solo mode except that everything has a lot more health, and the final boss fight has you facing off against a different opponent that is considerably harder than the end boss of solo mode.
Also, unlike in regular flashpoints, if you die, you can't simply pick things up right where you were defeated previously but instead wake up in a prison cell and have to go through a little sequence to free yourself and escape.
Similar to Kuat Drive Yards, the Star Fortresses have a modular design that is random every time you enter one. Excluding the boss level, the skytrooper production level in the middle (which is the same every time) and the two "transitional" levels which just consist of a short walkway, each instance consists of four floors you have to fight your way through, with each floor potentially having three different layouts as far as I could tell. Unlike in Kuat Drive Yards however, there are no different objectives to pursue here.
Example: The first floor could look like any of these.
There is little excitement to be had while fighting your way through the levels, as your opposition consists of Zakuulan troops and droids that pose little challenge. At one point in the instance, a bonus objective will light up, highlighting a certain room you have to clear out, which will earn you reputation with your currently active companion and with the one associated with that particular Star Fortress (see story section below). Also, one of the upper levels and one of the lower levels will contain a named "paladin", basically a hard-hitting Knight of Zakuul that serves as a sort of bonus boss. There are achievements for killing all of these.
Otherwise the only bosses you have to face are the Skytrooper Praetorian on the skytrooper production level, who uses various stuns and summons adds, and the instance end boss. On solo mode, this is the AI Ephemeris, which initially sics adds on you until you destroy its backup power cores, which results in it possessing a droid body that you can then kill.
On heroic mode, the final boss is the Exarch overseeing the Star Fortress (a special type of Knight), and you cannot face him or her until you've chased them all the way around the station's central sun reactor. This is quite challenging as the chase consists of several phases, each of which has the Exarch being guarded by two gold mobs, and more weak opponents continually streaming into the room, all while you have to interact with a console and sometimes kill a special add to move the encounter to the next phase.
None of the bosses drop "traditional" flashpoint loot - instead you gain lots of decorations as well as various item level 208 mods and enhancements, which is the same level as the gear you can buy with common data crystals. Additionally there are quite a few achievements that encourage you to re-run the place, including the ones to kill the paladins already mentioned, as well as ones to find lore objects or take on special challenges. If you get your influence level with your Alliance experts up to ten, there are also hidden caches on each level that you can unlock for extra combat abilities. I've previously written a bit more about some of these features here.
Not much story takes place inside each Star Fortress, however the story arc surrounding them is actually quite interesting. In KotFE chapter four Lana mentions that Arcann has had powerful battle stations built in orbit around several planets to keep their population in line. This is brought up again in chapter nine, when the actual side story arc for the Star Fortresses becomes available.
Planets that are being suppressed in this fashion are Alderaan, Belsavis, Nar Shaddaa, Hoth, Tatooine and Voss, and while you have a contact on each of these planets, you know too little about the interior of a Star Fortress to take action right away. You get tasked with exploring one of them to find out more, which means doing any one of them on solo mode first. While doing this, you learn that each Fortress is protected by a ground-based shield generator that needs to be destroyed before the Fortress itself becomes vulnerable (very Return of the Jedi-like).
After this, Alliance alerts for the contacts on the six planets pop up in your companion panel, and you can start each planet's individual story arc. They all follow the same basic plot: Your contact asks you to scope out the shield generator, then tells you that they and their allies don't have enough resources to mount an assault. You are prompted to hand in five resource crates of the type you get for doing heroics at max level, and then the attack can begin. Destroying the shield generator is trivial, and afterwards you are asked to destroy the Star Fortress in heroic mode. (You cannot start the heroic mode for any planet until you complete this part.) If you succeed, your planetary contact joins you as a permanent companion.
While the plot is very basic, what makes the story interesting are the interactions with your would-be companions. They are all gibberish-speaking aliens, presumably to save on voice acting, but many of them are given vibrant personalities and some very interesting conversation options. For example my Jedi knight was given the option to talk about her missing companions while chatting with Choza Raabat, the Ithorian Jedi who serves as your contact on Alderaan. (Though no, he didn't know anything about Lord Scourge's whereabouts either.)
That the Star Fortresses are labelled as flashpoints is a bit baffling and makes one wonder whether Bioware is trying to go into a new direction with this content type. Oh, to have been a fly on the wall when they came up with this... The fact that the solo mode seems to be the default and that the heroic mode is soloable as well initially gives the impression that this is meant to be a challenge that's primarily meant to be tackled alone. On the other hand the final boss on heroic is significantly easier in a group and well worth grouping up for, being more reminiscent in difficulty of the way heroic missions used to be. Also, the actual story arc can only be completed by doing all the heroic modes.
With all that said, the Star Fortress flashpoint occupies a strange place in the game, unlike any of the regular flashpoints. Locking access behind the first nine chapters of KotFE makes it quite hard to get into, and the lack of any sort of group finder option for it makes it even harder to get the heroic modes done if you can't or don't want to take them on alone.
It's not a good way of acquiring gear, however it's great for decoration drops and some of the better mods and enhancements can fetch a decent price on the GTN. Inside the flashpoint itself there also isn't much going on in terms of story or interesting mechanics (except for the heroic end boss), which makes them kind of grindy - which can be a good or a bad thing, depending on your preferences. Considering its story focus, SWTOR doesn't offer many opportunities for the kind of "mindless grinding" gameplay where you just mow down mobs for an hour while watching television on the side, but the Star Fortresses are actually really good for that. And the extra companions are fun, if not really needed considering the deluge of companions you end up with on your way through KotFE.
Finally I wanted to note that this marks the last installment of my Flashpoint Friday series. 27 posts and nearly a full year later, I've used it to talk about all the flashpoints currently in the game. I'm quite pleased with myself for having been able to stick to my fortnightly schedule for the entire time without missing a beat even once, but I have to admit that I'm also kind of glad that it's over. While I stand by what I said in the past about a series being a great way of providing yourself with something to write about at all times, it did start to overwhelm the regular content a bit when I started running the KotFE chapter discussions as well. Also, a full year is a long time to commit yourself to writing about a very specific subject like this - I suspect I'd think twice before doing that again.