The world of DARK•HERITAGE is one that, like our world, is subject to continental drift, which is a major force in describing how the land came to be as it is now. That said, some cataclysmic events that plate tectonics cannot explain seem to have had huge impacts on the world as well--dramatic shifting of continents on occasion, pole shifts, crustal displacement, and just plain old continents sinking under the sea (or rising from the sea) in a way that plate tectonics tells us is impossible--well it could happen here. It has in the past, at least.
Like our earth, the continent on which the Mezzovian Sea action takes place--the Mezzovian continent, for lack of any other label (its inhabitants don't really understand the concept of continent like we do, lacking a strong knowledge or feel for global geography) is an ancient landmass, but it's really made up of smaller terranes, cratons, islands and other pieces of crust that accreted and became sutured together. Evidence of this can be seen in the form of the old, hoary and worn-down mountain chains that cross north to south across the face of the continent (the Romeu and Garriga Mountains) that are not unlike the Urals, Caledonian mountains, Appalachians, or other older mountains that were formed when ancient continents collided and then remained fused together in new formations altogether. This ancient chain of mountains basically straddles the north and south shores of the Mezzovian Sea, and even extends underneath it; the Tolosa Islands are essentially parts of these mountain ranges that have their feet submerged in the water.
The Mezzovian basin itself is an aulacogen, or "failed rift" that sunk below the general level of the continental and became the floor of a shallow epicontinental sea. Because of this, it's not very deep, but there are pockets where the crust has collapsed to a surprising depth here and there in the distant past (millions of years ago, if not tens of millions). The shallow water and tropical/subtropical lattitudes combine to make the sea warm and pleasant.
Although subject to sometimes swiftly growing fierce storms, in general, the Mezzovian is known for being rather calm. Because of this, it has become a major highway for trade and conquest. The earliest group to develop a nearly circum-Mezzovian state was the Balshatoi, who were centered on the Razine peninsula. Their kingdoms and fiefdoms collapsed under economic and military pressure as the Terrasans surged northwards from their homelands in and immediately south of the Tolosa Isles.
While the Mezzovian Sea is a single large body of water, islands, peninsulas and bends in its own shoreline have created a number of "subseas" and other subsets that are important enough to have been named. The Novilda Sea is the westernmost; a great bight or bay that extends westward from the northernmost Tolosa Islands to reach the Erau, Urt and Volo river deltas. The most important regional power in the Novilda Sea is Segrià, one of the Terrasan cities.
To the very east of the Mezzovian, the sea pinches again to the rough and dangerous waters of the Sea of Storms, which in turns pinches even tighter to Shipwreck Strait, which leaves the Mezzovian and goes into the open ocean. Because the Sea of Storms, and even more Shipwreck Strait are difficult and dangerous to navigate, Qizmiran ships rarely pass through them, and Qizmiran sailors are divided between "ocean" and "sea" sailors; those that stay beyond the strait, and those who sail within the waters of the Mezzovian itself. This has also forced the Qizmirans to establish caravan routes that cross the Golden Peninsula and ensure that the inner sea colonies get the support that they need from the capital.