Final answer:
The Trelleborg Fortress was built by the Vikings, known for their advanced woodworking and engineering skills, which were essential in their extensive seafaring and trading activities across Europe.
Step-by-step explanation:
Trelleborg Fortress and the Vikings
The question relates to the Trelleborg Fortress, a structure built by Viking warriors on the island of Zealand. The Vikings were people of Scandinavian origin, known for their seafaring skills and expansion across Europe through both trade and conquest. The construction of the Trelleborg Fortress is consistent with the type of large scale and complex projects undertaken by the Vikings, who were also skilled in woodworking and engineering, as evidenced by their ships and buildings such as the Chieftain House at Borg.
Given the historical context and archaeological findings, the correct answer to which group of warriors built the Trelleborg Fortress using half the timber on the island of Zealand is a) Vikings. They were remarkable for their advanced seafaring skills, which they used to their advantage in trade and during their expeditions throughout the waterways of Europe.