These bastions aren't designed to be a tower-defense minigame.
Sometimes I miss my PCs could built "killboxes" like in Fortnite: Save the World ("Orcs must die" is more railroad).
We shouldn't worry too much because we can bet somebody will publish house rules for "grogs" or "demiPCs. And each grog would work like a room or special facility for bastions.
Other crazy idea is bastions to be "sentient" and even could being incarnated into avatars, something like "
genius loci". These would be like monster classes, but with a different leveling-up system, more "storytelling". We don't need it was too powerful, only level of a classic home fairy.
A shop, tavern or other type of business should be an easy access for potential customers, but usually these are in "commercial districts" within town walls. Not all the rooms or facilities should be together in the same place. Some facilities could be movile, like carriages.
* Other risk is when the DM wanted to play with the trope o haunted house. Some room or facility would be "locked" until the murder mistery was solved. Maybe this ghost later becomes a new setinel.
Not all the attacks should be a raid of gnolls or like this, but more a hunt by a supernatural predator, for example a theriantrope, or infiltration by a doppelganger spy, or a saboteur hired by a rival shop.