Portoga

In my opinion, the Kingdom of Portoga is where the History's Greatest Hits nature of Dragon Quest III really shows through. After clearing a dungeon in an area based on ancient Egypt, you unlock an area based on the Age of Sail. Nobody ever comments on it, or acknowledges that there's areas  coexisting simultaneously similar to those that existed in the real world thousands of years apart. Well, why not? Robert E. Howard didn't think twice about having Conan fight Egyptians in one story and 18th century pirates in the next, and neither, evidently, did Yuji Horii.

Portoga spans the area corresponding to the Iberian Peninsula. Its name recalls Portugal, but the capital city is located where Spain would be, so it isn't that specific. I personally wouldn't be opposed to throwing in some influences from renaissance Italy - scheming courts, powerful clergy, grand displays of the arts - and making it a catch-all for early modern Mediterranean Europe, seeing as how Romaly is solidly Roman.

A note on gunpowder: there are no guns in DQ3, and there is a bomb, but it's the creation of a lone alchemist and explicitly magical in nature. There are, however, cannon located on a ghost ship that serves as one of the game's dungeons, so there's arguments for both including and not including gunpowder in the setting. I'd personally go with gunpowder being unavailable (maybe with the exception of cannon) in order to keep the more ancient areas from feeling too out of place, and to preserve the sword-and-sorcery character of the setting, but Solomon Kane exists so that may not be a problem for you.

Portoga is a major naval power. It's known for its shipyards, and it controls territory on either coast of the sea to the south, with a lighthouse just north of the Isisian desert (incidentally, connecting to a portal that leads to Aliahan, suggesting it was built over an ancient Aliahanian ruin). It also has a strong mercantile character, trading with distant lands. The king is acquainted with a hobbit who guards the pass between Ashalam and Baharata far to the east, which implies the kingdom maintains many important trade routes, and is perhaps located at a nexus of them - it is, after all, quite close to both the Romalian Empire and Isis, and its ships would allow access even further beyond.

Spices are very valuable in Portoga - a bag of black pepper from Baharata is enough to buy a ship. This, incidentally, is a very good example of what Solomon VK of World Building and Woolgathering called "Vikings in Clown Trousers" in the comments of this post. That is, authentically historical details where you don't expect them. This is only one such example in DQ3; while Dragon Quest is now synonymous with a particularly idiosyncratic genre of Japanese high fantasy influenced heavily by anime, the earlier games at least were grounded in historical fantasy, and that's something I want to call back to with The Saga of the Ortegids. Incidentally, it reminds me that spices really need to be used as treasure more. In a pre-industrial society, spices from distant lands might well be worth as much as if not more than gold, and they might be harder to transport as well, as PCs would have to lug around bags and jars; it's not like they can stuff a handful of cloves in their purse. This could present some fun challenges for getting treasure back home.

Portoga would make a good home base for a campaign centered on trade, with the PCs as merchants venturing forth to distant lands. While DQ3 doesn't mention pirates being a problem in Portoga, its nature as a maritime trade power and its historical inspiration would certainly make it plausible for one to fight a lot of them in the area. Or you could combine the two, and have a pirate campaign set off the coast of Portoga.