The Algarve in summer is hot and crowded, but Castro Marim stays quieter than the resort coast. The big event is the Dias Medievais in late August — a four-day medieval fair that fills the castle and the town with re-enactors, jousts, falconry, craft stalls, and roasting pigs. It draws over a hundred thousand visitors and is genuinely well done; if you want to stay in town, book months ahead.
For birdwatching, autumn (September to November) is best, when flamingo numbers peak and the migration is in full flow. Spring is gentler and brings the wildflowers. Winter is mild and almost empty — fine for the castle and the reserve, less so for swimming on the coast nearby.