They get there about a stretch of time, but they aren't automatically replaced if missing in fight. Not surprisingly, a bard can develop a stronghold any time, but no followers get there right up until he reaches 9th level. As well as those you already get. A lot more, at https://dungeondragonsbard02467.blogdanica.com/13406424/a-secret-weapon-for-dungeon-dragons-bard