A Bastion is a location that belongs to a player character: a home, a stronghold, and a place of power that the character develops over the course of a campaign. A Bastion offers a character temporary refuge from the dangerous world of adventuring, and it provides opportunities for a character to craft magic items, conduct research, harvest poisons, build ships, and carry out a range of other activities.
As DM, you decide whether Bastions are available in a campaign. Bastions are best suited to campaigns that allow characters to return to their Bastions during intervals when they’re not actively adventuring. Not every character needs to have a Bastion. It’s fine for some players in your campaign to opt in to Bastion ownership and others to opt out.
There’s no need to choose between going on adventures and commanding a Bastion; a character can do both at once. A Bastion has special facilities that generate benefits, and these facilities can also undertake projects while the character is otherwise occupied.
Most importantly, a Bastion is a creative playground for a player and a shared storytelling space in the campaign. Be as permissive as you can with the stories players tell in their Bastions, but players should know their control might be limited by the campaign’s larger story, which you strive to make fun for everyone.