As keepers of Rotties we have gone through the process of registering them. Each town hall in the area have different dogs on their lists, for example our town hall (Freginals) uses the list from St Carles de la Rapita, which has Rotties on it but not Alsatians, which we also have.
The process is very straight forward, but as usual time consuming - you need the following if required to do it.
Dog to be microchipped and vacs up to date
A certificate from Madrid central police to state you have no criminal record (The form for this is available at the local court, cost is 7.60 euros) Any criminal record will not allow you to keep the dog
Owner to undergo same test as needed for a driving license, and a 90 question psychometric test (in Catalan), from the same place as the driving license medical, cost about 60 euros + 2 passport photos of the owner
Public liability insurance for the dog (for us to the value of 150,000 euros, but each place is different)
Registration form from the local town hall
The dog still has to be muzzled in public places. If you want the dog to go unmuzzled more testing of the dog will need to be completed. The license then lasts for 5 years. If the dog dies a death certificate must be provided to the town hall as soon as possible after death.
So for those who know me, now know I am crime free and of sound mind. Not so sure about my dog at times though