Puns aside, the arrival of the Naban on the shores of Japan signals the arrival of a powerful new trade partner, deadly new weapons, and a strange new religion. Whether you choose to embrace, grudgingly accept, or renounce the ways of the Naban their arrival can have a massive impact on your campaign to become Shogun of all Japan.
Just as a smart military commander will look to bolster their advantages and minimise their disadvantages, a smart Daimyo will look to take advantage of the Naban while minimising the unrest trade with them can bring. Through the strategic construction and placement of Buddhist Temples, and the recruitment of Buddhist Monks, a smart Daimyo can negate the negative effects the Naban Trade Port brings.
As with many facets of Shogun 2 the Campaign Difficulty setting can influence how easy or hard it will be achieve your goals. This is no different when counter-balancing the negative aspects of the Naban Trade Port, as campaign difficulty also affects the Religious Zeal. Which, for the uninitiated, is the rate at which the Naban Trade Port converts your populace to Christianity. For the purposes of this article we will be assuming the use of Normal campaign difficulty level.
The Naban Trade Port offers many advantages to a clan willing to build one within their domain - unlocking the ability to build Imported Matchlock Ashigaru, maintain two trade routes out of the same port, massively improve town growth, and export capacity (trade value). The one, and obvious, drawback to the Naban Trade Port is the fact that it can convert your population away from Shinto-Buddhism to Christanity.
Cancelling out this disadvantage is simple in theory, but potentially quite hard when it comes down to practicalities. Depending on the counter-strategy adopted the Naban Trade Port will tie up building slots, Monks, or both as you equal or better the Religious Zeal of the Naban Trade Port. Usually an equal or greater amount of Shinto-Buddhist Religious Zeal can be achieved through the building of a Buddhist Monastery, an upgrade to the base level Buddhist Temple, and the recruitment of a Monk who produces two Religious Zeal for use within the region.
As you can see there is a lot of room for change in this strategy if you stack the benefits of Monks, use a Monk who excels at converting the populace, or quickly upgrade your settlement to include a Buddhist Temple Complex. Another more complex strategy acknowledges, and takes advantage of the fact, that Buddhist Monasteries, and higher Temple upgrades, spread Religious Zeal in to neighbouring regions. Assuming you can build your Naban Trade Port in an applicable region, the Religious Zeal of neighbouring Monasteries or Temple Complexes could even cancel out the Religious Zeal of the Naban Trade Port right from the start. Perhaps surprisingly, it only takes one applicable neighbouring Buddhist Temple Complex to negate a Naban Trade Port.
With Christian Religious Zeal now under control rest safer in the knowledge that with the Christian population unlikely to grow that in times of unhappiness Christian rebellions will be less likely to form. Now go forth wise Daimyo and exploit this whole Naban situation for all that it is worth - and remember to consult the Encyclopedia, settlement or agent report, and in-game tooltips for advice as to how much Religious Zeal something produces when looking to counter the Religious Zeal of the Naban Trade Port.