I work as an SDE for Amazon in Madrid since 3 months ago, having relocated from Buenos Aires for the job. If you are cut for the culture, I can't recommend it enough. Everyone is fluent in English, and all work happens in English. I read really bad things about the work environment at Amazon before joining, but I guess that must be in the past or in some other office because I have seen nothing of the sort here; the environment is demanding but reasonably so. They pay is (much) higher than anything else I've heard of in Spain for a developer, and the relocation package was very generous and helpful.

If you are interested, here's a job search: https://www.amazon.jobs/location/madrid-spain?base_query=&lo...

I will echo what others said though: you need to speak Spanish in Madrid in order to survive, more so than in Barcelona. Outside of the more touristy areas English may not be enough in many places.