The hill of Lamia has been fortified since antiquity, and the current walls — a layered accumulation of Macedonian, Frankish, and Ottoman construction — wrap around the summit above the modern city. The views from the battlements across the Spercheios valley and toward Mount Othrys are excellent and well worth the uphill walk.
Inside the castle precinct, the Archaeological Museum of Lamia is a compact but well-organised collection covering the Phthiotis region from prehistoric times through the Roman period. Highlights include finds from the ancient city of Halos, a Bronze Age settlement in the valley; a good collection of funerary stelae and votive reliefs; and material from the wider Spercheios valley that puts the Battle of Thermopylae in geographic context. The museum is small enough to do justice to in an hour. Admission is inexpensive and the castle grounds can be explored freely once you have a ticket.