The Bulwark is a massive wall that spans across several southern regions. There are multiple gateways and breach points along the large wall. The Bulwark also has an interior at certain sections which can be accessed via rubble or stairs.
Not much is known about the construction of the Bulwark itself. However, it is known that construction started 200 years before the start of the Great Wars. The motivation to construct the Bulwark was to defend the nation from southern invaders (namely Meseans). Their puppet state of Veli was unstable, and it was assumed that civil war inevitable. The Meseans would support anti-Caoivish rebels, so as to protect their interest, they began construction on the wall. However, the construction would cut off Veli from Caoivish aid, which led to ignition the expected civil war (and ultimately the Battle of Red River).
Considering the route the wall follows, it can be assumed that the wall was constructed on what was at that time the border of Caoivia (otherwise, it would have followed rivers and build away from the cities). Interestingly, the Bulwark also encompasses the dam in The Drowned Vale, which was later destroyed to flood the area.
At some points, a Colonial force invaded from the south, by destroying parts of the walls, in an event known as The Breach. Various sources refer to this, from various view points.  It is a major event at the start of the Great Wars, which resulted into the invasion of the Colonial army into the in-lands of the Warden nation, with the retreat of the Warden army to strongholds and fortifications further back.
In-game, the ruins are an indestructible structure in the landscape. It can be found in Westgate, Heartlands, Umbral Wildwood, The Drowned Vale and Allod's Bight. There are various sections destroyed as to allow passage through the wall, and a few sections where the wall has completely disappeared to allow vehicle access. On a few sections, mostly in the cities it passes, it is possible to access the wall; the roofs will become transparent as to navigate the corridors, and there are windows through which can be fired.
The gate of the main road through Blemish
- "While Legate Silas Maro was the first to walk the crumbled remains of the Bulwark following the Warden army's cowardly attack, it was his descendent, Thea Maro who led our Legions through the hostile north.[...]" from the Inner Haven Volumes.
- In accordance if the lore text of The Battle of Red River demo - read the original at Lore_References#The_Battle_of_Red_River.
- "They attacked! The southerners actually attacked! [...] We'd hoped the Bulwark would keep our lands separate [...]" Found on a skeleton in Deadlands - Lore_References#Letter_found_on_a_Warden_skeleton