Yes Spain does have a land register I think it is in 2 parts the “Cadastral” is a very sophisticated co-ordinate based map of the whole of Spain which breaks the total land areas down to “Parcelas” mine is around 1000sq m and is clearly identified by a parcel no R6-**. This can all be looked up on-line. The second is a paper based system located in the Land Registry office of your local town and you can walk in and ask for information on a particular property fill in a form with a reason for wanting to know (such as property purchase) pay a fee collect the papers then or later and you will get all that is registered about that property.

All derelict property does not necessarily not have an owner or is for sale. Spain has a huge diaspora over many generations and because of the forced heirship rules after a couple of generations a property could be owned by 20 or more descendants some of whom may be in South America or Canada and their whereabouts not known. Those still in Spain may want to sell but cannot because all owners need to sign the transfer! In my Town of Ayamonte I know of at least two cases like this one of which was being chased by a Spanish lady who knew all the rules but could not crack the USA inheritor. It still stands derelict in the middle of a very desirable street.

I think the local council may now have power to buy-in such property but it does not seem to be used round here.

if it’s a small village there may still be a chap who would be the forerunner of the Gestor, he would know.

it may seem like nowhere to you but some Town will cover the area. Spain is huge with vast areas of campo with very few house but they do belong to one or other municipality. Ask at the hotel which is their Town Hall. This may not be big enough to have a Cadastral office but there will be one in a bigger town nearby.

if it is so remote you may find the ruin comes with a couple of sq km of land as well is that what you want?