The Moors conquered it in 715. Ordonho II took it from them in 985 for the Christians but lost it soon after. In 1037 Fernando Magno conquered it but he too lost it. Finally in 1132 King Afonso Henriques conquered it once and for all and gave it a charter in 1136 under the name Civitasem Sennam. This charter was confirmed in 1188. King Afonso II gave it a charter in 1217 and King Manuel I in 1510.
King Afonso Henriques built a castle but not even ruins are left. It is presumed it was built on the site where the Parish Church stands. The castle was proof of the military importance of Seia, which was agricultural. This lasted during the Middle Ages.
King Manuel I elevated Seia to a county, reinforced the walls and built several churches. In the 18th century it became a textile centre and the town grew and prospered. There are still some Baroque façades from this era. It has been a city since 1986 and depends on agriculture, sheep, farming and textiles. It is also the main access to the Estrela mountains.
One of the main entry points to the Natural Park of Serra da Estrela, the ancient town of Civitas Sena leads to several main sites of the mountainous range, namely to Sabugueiro, the highest village in Portugal, on the way to the Tower (at almost 2000 metres, or 6100 ft), and to the deep and glacier-cut valley of Lorigo and Alvoco. Several monuments, namely manor-houses, confirm its important role in the past. It is worth admiring the Romanic Chapel of St. Peter, rebuilt in the 16th century, the Mother Church (with stunning views over the surrounding landscape) and the 18th-century Miserichord Church.At the nearby village of Valezim, there are three old water mills of a type not often found in Portugal, and two of them are still used to grind grain.
In the region, the eroded granite of the mountain tops have provided many weird shapes, such as the "Cabeça do Velho" ("Old Man´s Head"), near Sabugueiro, matched by the "Cabeça da Velha" ("Old Woman´s Head"), just south of Seia. As in other zones of the "Star Mountain", visitors can delight themselves with the buttery and rich-flavoured Serra cheese.
The Nature Reserve, with its paths offering beautiful views and stunning flora, never cease to attract walkers and nature lovers.