Arabic borrowing in Spanish and Portuguese

Can anyone explain why so many Arabic words, which were adopted by
Spanish and Potuguese, such as alcazar, alcalde, almohada etc retain
the Arabic definite article 'al'. I know this is not universal and
there are many exceptions such as 'zanahoria' but a great many (the
majority?) do retain the definite article. This means that when
someone says 'el alcazar' he is really saying 'the the fortress'. Any
thoughts?
Sorry about the absence of stress accents.
Regards

David James