Currently Silgrod Pass, formerly Septim's Gate Pass (I think).
This is a reimagining of an existing exterior. It is currently implemented as an Imperial border fortification. Landscape edits which are limited to the four cells in this claim are allowed.
Knocker's Neck Pass borders the Orcish tribal lands of Marahk-Bazhul, which are nominally part of Morrowind on Imperial maps. In practice, they are tribal, independant (Switzer)lands inbetween civilisation that nobody wants to bother to conquer.
The Cyrodiil end of the border is still in Imperial hands, but Knocker's Neck Pass has passed from the Reman Empire (who built an observation post here) to either House Redoran or the Buyoant Armingers. This is at the discrection of the claimant. Trade caravans pass through here, but are usually accompanied by a Redoran champion whose role is to challenge Orc raid leaders into single combat. As such, it should have accomodations for travellers and be actively manned. It has not fallen into disrepair and ruin.
Two very rough concepts exist. If the pass is manned by House Redoran, it should use the appartment-style buildings of Kartur, forming a ribcage inbetween the mountains.
If it manned by the Buyoant Armingers, it should be in Velothi style.
The actual inside could run deeper into the ground, to make up for lack of exterior space, maybe alluding to a cockroach hive, writhing under the surface.
As for the function of the tower, it has always stood by itself, serving more the role as a sentinel tower, a checkpoint, and perhaps housing a small garrison, than really blocking the pass. When the Reman Empire built the tower, it was as a lookout. The actual war with Morrowind had already moved on deeper into the province and they were probably more concerned with stragglers or guerillas.