During the 16th century the naval ships were developed with increased size and a larger quantity of guns. When the Swedish ship Mars exploded and sank during action in 1564, she was one of the largest ships in the world. In 2011 the wreck was relocated outside the island of Öland in the Baltic Sea. Thanks to the favorable conditions about two thirds of the hull are preserved which includes the stern with the large stern castle. The site thus offers a unique opportunity to study a state of the art warship from this period. The site has been surveyed during three seasons. This paper is based on these initial surveys and aims to present the kind of insights regarding 16th century naval architecture that Mars has revealed so far.