Fuerte de San Cristóbal was the first major work in Badajoz's modern bastioned defences, begun in 1641 on a hill across the Guadiana. Its job was to guard the city from the right bank of the river, and the fort still makes more sense when you look back across to the Alcazaba and the old centre.
The restored complex includes an interpretation centre for the border fortifications, but the broader appeal is the military landscape: river, city walls, exposed ground, and the long Spanish-Portuguese frontier story. It is slightly apart from the compact historic centre, so plan it as a deliberate detour.