During the Viking age they didn't have explosives. That's why they used catapults. They didn't have beasts of burden until after they started raiding. Their area of the planet had reindeer and deer. They had to build everything by hand with ropes, pulleys, and hand held tools.

Catapults could destroy stone walls but not thick earthen walls. To destroy earthen walls you had to tunnel under them and then set a fire in the tunnel to get the earthen wall to collapse. That's why Viking trellborgs were ringed by earthen walls. The tools they used to build earthen walls would have been shovels, hoes, and manual tampers. In game the hoe stands in for all of those things and time is compressed because a single viking wouldn't have been able to do all that in his lifetime.