Castle of Mourão

The castle is made up of a medieval wall, built in a curious combination of schist, marble and granite stones, reinforced by six quadrangular towers. The gates are flanked by turrets, some with pointed arches in the Gothic style. Among the towers, the Gothic-style keep stands out, with a door opening onto the parade ground. Inside the enclosure, there are the remains of the Guard House and the old Town Hall. The 17th century renovations included bastions at the four corners of the wall and ravelins in front of the curtain walls.

The castle is free to enter and has parking just outside the main gate.
It's a bit out of the way but you can see it on the hilltop for quite a distance and it is in very pretty countryside.
The outer walls are intact and that's pretty much all there is. There are almost no buildings inside and you can't climb on the walls. There is no information other than a plaque by the church built into the walls. Good views though as you walk around.

Mourão Castle is well-preserved and can be visited free of charge. You can climb the castle walls via an adventurous climb (they're dilapidated and completely unsecured).
Pacmen64 🇵🇹 isn't right here, although I otherwise greatly appreciate his contributions.