Surprised by such easy gains in the East, the Republic quickly decided to exploit Portugal's weakness in the Americas. In 1621 the Geoctroyeerde Westindische Compagnie  was created to take control of the sugar trade and colonise America . The Company benefited from a large investment in capital, drawing on the enthusiasm of the best financiers and capitalists of the Republic, such as Isaac de Pinto, by origin a Portuguese Jew. The Dutch West India Company would not, however, be as successful as its eastern counterpart. The Dutch invasion began in 1624 with the conquest of the then capital of the State of Brazil, the city of São Salvador da Bahia, but the Dutch conquest was short lived. In 1625, a joint Spanish-Portuguese fleet of 52 ships and 12,000 men rapidly recaptured Salvador. In 1630 the Dutch returned, and captured Olinda and then Recife, renamed Mauritsstadt, thus establishing the colony of New Holland. The Portuguese commander Matias de Albuquerque retreated his forces inland, to establish a camp dubbed Arraial do Bom Jesus. Until 1635, the Dutch were unable to harvest sugar due to Portuguese guerrilla attacks, and were virtually confined to the walled perimeter of the cities. Eventually, the Dutch evicted the Portuguese with the assistance of a local landlord named Domingos Fernandes Calabar, but on his retreat to Bahia, Matias de Albuquerque captured Calabar at Porto Calvo, and had him hang for treason. The Portuguese fought back two Dutch attacks on Bahia in 1638. Nonenetheless, by 1641 the Dutch captured São Luís, leaving them in control of northwestern Brazil between Maranhão and Sergipe in the south
Question: What were the only safe areas for the Dutch to harvest sugar?
Answer: walled perimeter of the cities