|A 1570 Flemish map of Formosa|
How did the Kingdom remain powerful, not only had it survived the Chinese embargo but also prospered to become the hub of East-West trade in its short 22 years of existence?
The famed traveler and writer 郁永河 (1645-?) had answered this question in part. In his report 「偽鄭逸事」, he stated that "成功以海外島嶼，養兵十餘萬，甲胄戈矢，罔不堅利，戰艦通洋數以千計。又交通內地，遍買人心，而用財不匱者，以有通洋之利也。本朝嚴禁通洋，片板不得入海，而商賈壟斷，厚賂守口官兵，潛通鄭氏，以達廈門，然後通販各國。凡中國諸貨，海外人皆仰賴鄭氏。於是通洋之利惟鄭氏獨操之，財用益饒。" Translation: "Koxinga commanded a fully-equipped military force of more than 100,000 men, and a fleet of several thousand warships. He was also able to communicate with people on the Mainland and enjoyed their loyal support. His inexhaustible resources came from the foreign trade. Even though our gov't (Qing) had banned the trade entirely, allowing not even a single ship to put to sea; however, rich businessmen would bribe the border-guarding officials/soldiers to secretly deal with the Cheng Clan in Amoy, and through which to trade with foreign merchants. The distribution of all merchandises treasured by the foreigners must go through this Cheng monopoly, and Cheng became even more wealthy as a result."
There were actually several episodes of Qing sea trade bans, all intended to cut off the economic lifeline of Taiwan. These never really worked, however.
The first ban was imposed in 1657 (Qing 顺治Sun-Zhi Year 13) that prohibited coastal area residents from any contacts with the Cheng Clan, on pain of death. When this had not achieved the desired goal, a far more draconian measure, urged by another Ming-Cheng turncoat 黃梧, was enacted in 1661. An even more drastic version followed in 1664. And the last known reinforcement amendment of the law was declared in 1678. Under this cruel law, residents within ca 17 to 34 km of the coastal areas in 5 provinces, 山東, 江(蘇), 浙(江）, 閩（福建）, and 廣（廣東 and 廣西）, were forced to relocate inland. Again, refusal to comply was punished by death. Hokkien, the homeland of the Cheng Clan and their followers bore the most brunt. All together, the affected were in the millions, not to mention the loss of lives and properties. With the latter, also the loss of tax revenues which were perversely compensated for by raising the taxes of those not under the law. In 1664, Berlin-wall-like structures complete with moats were built and staffed with garrison forces to stop any and all traffic and trafficking. In addition, all residents of offshore islands were also forced to relocate behind the walls.
Far from buckling under these pressures and instead of relocation, coastal people fled to join the Ming-Cheng force in Taiwan. And the Tung-Ning Kingdom continued its foreign trade tradition and amassed a huge fortune for Cheng Ching to mount another attack, as his father had done before him, on China in 1674.