stronghold 1 never had AI enemies, skirmish AI was added to the sereis for the first time with stronghold crusader back then, the definitive edition for stronghold 1 does not have skirmish AI either because the team that made it was too small to backport skirmish related code from crusader in the timeframe they had to make the definitive edition for stronghold 1

edit: in the campaigns you either have completely passive castles to storm or completely scriptet attacks to defend from and those attacks are spawned in, there is no AI capable of building a castle or recruiting troops like skirmish AI in the rest of the stronghold series can (with the exception of stronghold 3 since that also has no skirmish AI)