The mast is probably a single, large tree, and not a 'made spar' comprising of several scarphed pieces of wood. It is stayed (held up) by taught lines running from the sides and front of the vessel. These stays or shrouds are shown without rope rungs running across them which would have helped the crew ascend the mast.
This may be a correct interpretation of depictions on seals (where a great deal of the information about ships of this period comes from) as these sources never show these - although there is a good argument to be made about the level of detail that can be incorporated onto these small carvings.
As it is, some pictures clearly show cumbersome objects (like crossbows) being used from the fighting position on top of the mast; and it seems likely that some easy method of getting these up there must have been used.
It has a 'square' sailWhilst Mediterranean shipwrights built lateen (triangular) rigged ships, square sails had always been used in northern waters. Even though travel between these two major ship-building areas had been common for many hundreds of years, it was not until the late 14th Century that the first triangular sails (set fore and aft) were incorporated into northern-built ships; and only then to add to the effect of square sails and to improve the ship's abilities in terms of sailing to windward.
With this improvement in sailing ability, it might be wondered why the lateen rig wasn't adopted in northern Europe, but the answer lies in the fact that, for best results, tacking a lateen-rigged ship (turning through the wind) requires the entire rig of spar and sail to be changed to the downwind-side of the mast - a time consuming process. For a square sail during tacking, the yard is just pivoted about the mast.