Q:Why can't I build any ships in the Caspian area?
A:Because historically the area was more an amalgam of mercantile and commercial interest as well as adding to the fact that the northernly reaches were sparsely populated steppes, which further diminished the competitive factors and rather bolstered local enterprises. While there was certainly a local industry for building transporters and such to a lesser extent, and some scant trade by sea, no Parthian city in the area supported any significant port, and there was no economically competitive nor significant factor in the northern reaches, thus as a consequence there is no reason to expand any industry of transporters beyond local needs, let alone vessels of war or to drastically change economical prospects that historically remained the same for several centuries. Out of all the great waters, the Caspian Sea, beyond rich stocks of fish (The famed Persian sturgeon), and in spite of rich coastal areas, in particular the fertile Albania and well-watered Hyrcania famed for her dense forests, clearly did not carry the same potential as the Black Sea or the Persian Gulf. Add to the the fact that the Sarmatians to the north did not share the same taste for commercial enterprise as the Parthians to the south. Hence, the Caspian area is not very lucrative at all in matter of trade. You can still build sizeable places of trade, but not beyond the possibilities for expansion, let alone a base for a navy.
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