María Luisa,

It's clear that this kind of construction is quite specific to Spain. So as I see it, I'd leave the term as is and add an explanation of the exact characteristic of the monument you're translating for.

These are other definitions I found, and even in Spanish they are long to explain:

Coracha:
♦ Paso protegido por una doble muralla, entre un cerco urbano y un castillo, torre o atalaya.
http://es.wikipedia.org/wiki/Coracha

♦ muro. Prolongación hacia el exterior de la muralla de una fortificación que puede ir unida a una torre albarrana independiente del recinto fortificado.
http://mf0.op.upm.es/avec/glosario-arq.htm

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So, my suggestion would be something like:

♦ Coracha (double wall descending to a river for water supply)
♦ Coracha (a small section of wall that ends in a tower)
♦ Coracha (a walled passage which links the castle with the Alcazaba as a form of protection )

And in brackets the specifics of your particular coracha.

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Some examples:

MWNF - Museum With No Frontiers
The Gibralfaro Castle, linked to the citadel by a coracha or double wall, was built under the Nasrids to reinforce the defensive system. Related Dynasties ...
http://www.discoverislamicart.org/pc_item.php?id=monument;IS...

Know, Sleep and eat in malaga- Monuments in Málaga
... that has been built on top op the Gibralfaro hill, near the Castillo de Gibralfaro, with which it is united by means of a double wall called coracha. ...
http://www.doitinspain.com/infociudad/English/malaga-monumen...

Spain - Trip Planner - Yahoo! Travel
... but it was Sultan Yusuf I of Granada who had the Coracha (a walled passageway that ... Pere Bonfill as an entrance through the medieval defensive wall. ...
http://travel.yahoo.com/trip?nr_cat=Things To Do&pid=598581&...

CSIC: Escuela de Estudios Árabes - The Albayzin
The boundary then descended once again along the river to connect with the coracha (wall descending to a river for water supply) of the primitive enclosure, ...
http://www.eea.csic.es/index.php?option=com_content&task=vie...

Islamic Madrid
The absence in the wall of a loophole, or barbican, is made known to us by tradition, and by the existence of a "coracha," a small section of wall that ...
http://www.nova.es/~jlb/mad_in09.htm

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Note added at 31 mins (2007-04-02 00:51:50 GMT)
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(these, of course, are mostly Spanish websites, but that's the point: something so specific should be left as is and explained)