No, it's not. Bastion defenders would be a roleplaying choice
Barracks have an opportunity cost that is much higher than any benefits you might gain from them, and efforts to improve them are likewise unsound. Here's why:
There is really only one instance in the Bastion rules that leads to an attack on your bastion: the "Attack" Bastion Event, which can only happen when the bastion is under the Maintain order (because the owner issues that order, or as a fallback because they cannot issue any orders during the bastion turn because they are away and have no way to contact the bastion, p. 336 PHB).
The Attack event can actually not destroy your Bastion. The only things that can lead to the fall of your Bastion are listed on p. 352, under Fall of a Bastion:
A player can lose their bastion in the following ways:
Divestiture. A character can give up their Bastion anytime, releasing the Bastion's hirelings and abandoning the location. [...]
Neglect. If a character issues no orders to their Bastion for a number of consecutive turns equal to the character's level [...], the hirelings abandon the Bastion and the site is eventually looted. [...]
Ruination. Drawing the Ruin card from the Deck of Many Things [...]
That is it. Attack on the Bastion is not one of them, you can never lose your Bastion to an attack under the Bastion rules. Instead, the Attack event works like this:
A hostile force attacks your Bastion but is defeated.
Roll 6d6; for each die that rolls a 1, one Bastion Defender dies. Remove these Bastion Defenders from your Bastion's roster. If the Bastion has zero Bastion Defenders, one of the Bastion's special facilities (determined randomly) is damaged and forced to shut down.
A special facility that shuts down can't be used on your next bastion turn, after which it is repaired and made operational again at no cost to you.
So the worst that can happen as a result of an attack is that you cannot use one of your special facilities for one turn. Either because you had no Defenders to begin with, or because all of them got killed in the attack, so you likewise have zero Defenders.
Normally, your Bastion has no bastion defenders. To get them, you need to pick the Barracks special facility, which then allows you to issue the Recruit order and hire up to 4 Bastion Defenders, to a maximum total of 12 of them. (You can also get them from a Menagerie, later on).
Unfortunately, mechanically this is a waste of resources, for this simple reason: an Attack event only happens on a roll of 51-55. That means if you have no Barracks and no Bastion Defenders whatsoever, you lose one use of a random special facility with 5% probability (or less than once every 20 turns on average, if you order anything else but Maintenance from time to time). However, just by taking the Barracks, you lose the use of one facility every Bastion turn that is not Maintenance, because you now have a Barracks that does nothing if you are not attacked instead of another facility that can do something. It is much better to just not have a Barracks and lose one use of a facility once in a blue moon, than lose a use of a facility every Bastion turn where you want the Bastion to do something.
Is improving your Barracks worth it?
But lets assume you want a Barracks. Should you spend something to improve it?
The Bastion rules offer a manner of ways to do that, for example:
- Spending 2,000 gp to expand the Barracks to house 25 Bastion Defenders
- The Menagerie allows you to Recruit creatures that can act as Bastion Defenders
- The Armory allows you to spend 100 gp plus 100 gp per Bastion defender, and to then roll d8 instead of d6 in the Attack event (lowering the chance of losing a defender, as a 1 now only happens in 1 of eight outcomes on each dice, instead of in six).
- Building a wall for 250 gp per 10 foot of wall to completely enclose your Bastion, in which case you lose 2 less Defenders in an Attack.
- A War Room has Lieutenants, each of them reduces the number of dice rolled in an attack by one.
It likewise seems like a royal waste of resources to try and further harden your Barracks or defenses in any way. If you have the Barracks, its basic form will all but ensure you never run out of Bastion Defenders. In an Attack, with the unmodified 6d6 roll you expect to lose only one defender on average. The chances are vanishingly small that you would lose all your defenders to an attack. It can only happen if you have six or less defenders, and even the basic Barracks allows you to stock 12. You also can restock them much faster than you lose them: you expect to lose one defender every 20 Maintenance turns, and if you cannot issue Recruit orders for 20 turns, you will lose the Bastion to Neglect no matter what. You can recruit up to four each non-maintenance turn, so the chances of ever dipping to six or less once you have stocked up to 12 are essentially zero. Your simple Barracks alone, with its stocking capacity of 12 Bastion Defenders, will mean that you'd only lose all of them in an astronomically unlikely series of events - and even if that happens, it doesn't really hurt that much.
That means, if you chose to build a Barracks, then certainly spending hundreds of gold pieces to equip those soldiers with weapons or to build walls around to lower their losses is merely a waste of further resources.
So in my mind, it only makes sense from a roleplaying perspective to spend resources on a Barracks, on enlarging it, on walls or on an Armory instead of, say, an Arcane Laboratory which can produce uncommon magic items for you or a Library, which can provide you with information you otherwise would not have. (The War Room has other benefits that may make it desirable).
Maybe if the DM handles the Bastion not just with the Bastion rules, but instead is playing out normal attacks from normal NPCs on the Bastion and its hirelings and Defenders, it might make sense, but that is outside of the purview of the Bastion rules as written.
