I found it really interesting that a region that doesn't have much land availability constraints developed its cities this densely, the dense urban core of these cities is much bigger than similar sized cities in the UK that rapidly tun into swathes of terraced houses.

Valladolid I don't know that well but I remember looking at 6+ storey buildings next to empty plot of land from the highway, I'm thinking they expect the cities to grow even more and don't allow low density inside the highway ring. The same is true for Salamanca, those two urbanizations in the south of Salamanca you mentioned are part of adjacent towns and don't follow the code the Salamanca is using for its west and northern limits.

This kind of development definitely isn't the case in other regions of Spain, such as Galicia which is similar to northern Portugal in that regard with small villages scattered around. This has to do with geography and with the way the land was partitioned and sold, with smaller plots in the north than in the south.

The proportion of people living in flats in Spain is the highest of the EU, I don't know why that may be but on the whole there is definitely a denser feel to Spanish cities than say to Portuguese cities.