请教tpo43 阅读里面的一句话，怎么理解？

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Towns and cities were established not only as garrisons (military posts) but as centers for the diffusion of Greek language, literature, and thought, particularly through libraries, as at Antioch (in modern Turkey) and the most famous of all, at Alexandria in Egypt, which would be the finest in the world for the next thousand years.

我有点看不懂后面半段的结构。是不是说，towns and cities 建立不仅作文garrisons, 还作为Centers传播XXX，尤其通过图书馆... 而后面的两个是图书馆的举例。是这样子的吗？请大神指教。谢谢