Way back when, I drew more than a couple of maps of underground fortifications. They were undoubtedly heavily influenced by the original D1 Descent into the Depths of the Earth adventure and the others in that series. In many cases, I conceived of guardposts like this being fortifications built by dwarves or humans, keeping in mind that this was long before the in-depth writings and background on the Underdark and on things like the architecture of Drow cities.
The first two maps are a completely underground set of fortifications with the upper levels excavated beyond the main tunnel blocked by the fortifications. The third map is a fortification at the mouth of a tunnel to the outside that could lead to a settlement that could either be completely underground or above ground and only reachable by a tunnel (such as within the caldera of an extinct volcano or atop a rocky tor).
Underground redoubt 1 main level
Underground redoubt 1 upper levels
Underground redoubt 2 main level