Many believe this structure to be a GDI Starport.
Being that GDI maintained full orbital dominance, the concept behind this structure is valid.
However, considering that GDI Dropships were large enough to carry towering Mammoth Mark IIs, this structure and the subsequent base required to defend it must have been massive, overly costly, and strict in their selection of specific terrain where the structure could be effectively built.
Knowing these factors, I believe this structure may have been scrapped because, realistically, it was unnecessary considering Dropship could land without this Starport.
Note the concept GDI logo featuring missiles on the control tower on the left side.
This Starport has been featured in a Tiberian Sun concept video in which a Dropship lands and deploys a Mobile Construction Vehicle out of the side door of the Dropship's underbelly cargo pod. However, the Dropship in that case is significantly smaller than in-game concerning realism.