actually the Cardan-iii are essentially an upgrade package of the Cardan I and II. larger than the two previous they were designed to go from one level to the next rather easily and to be easily assembled. The cardans came in right at the end of the clone warsAdded (20 Jan 2011, 9:06 Am)
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The Cardans themselves were quite frequently deployed by the Empire with various stages not like a ship which is improved the Cardans were built with different models to be deployed in different areas.
Added (20 Jan 2011, 9:23 Am)
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to further prove my point this is copied from Wookiee:
These stations were first designed towards the end of the Clone Wars, as a means of protecting Republic worlds against encroaching CIS fleets. Several prototypes were built, and production got underway, but deployment of these defense stations didn't happen before the Republic had been replaced by the Galactic Empire.