Spanish is an old language, it started to develop from vulgar latin with the fall of the roman empire. After the Arab invasion in the north of Spain began to form so-called Romance languages, all derived from Latin, one of them was Castilian, named after the region it came from, Castilla, and this language was extended during the reconquest of the territory lost to the Arabs.
Castilian and Spanish is the same thing?
Yes they are, the original name was "Castilian" but perhaps now is more common to call it "Spanish" but both are valid.
Spanish around the world:
As you probably already know Spanish is one of the most used languages in the world, as the second language after Chinese and English ahead. It is also the second most studied language after English, more than 20 million people study Spanish, some in appreciation of Spanish culture, others because of it's importance in the world and others simply as a hobby.
Is it easy to learn Spanish?
Well, everything depends on the person, his innate ability for languages and especially the proximity of the student's mother language with Spanish language, as you know the Spanish comes from Latin, so French, Portuguese or Italian people for example will find it easier than someone whose language is Germanic or Slavic, for example.
As a general rule considering my experiences with different languages and people of different nationalities believe that Spanish is quite simple language to learn.
How soon will I speak Spanish?
Like the previous question depends on each person but I would say that spending about 4 hours a week you can achieve good results, for example after a month or so following this Spanish course you will be able to manage simple conversations like presentations, ask where the hotel is or buy a train ticket.