The phrase "although not built" is correct and usable in written English.
It can be used to introduce a contrast or exception regarding something that has not been constructed or established.
Example: "The bridge was impressive, although not built to withstand heavy traffic."
Alternatives: "even though it wasn't constructed" or "despite not being built".
Although it was not built by the Druze, shepherds from the tribe were the keepers of the fortress and the first to call it Qal'at Namrud, after the Biblical hunter Nimrod.
Although it was not built on particularly high ground the inner castle, which occupied much of the area of today's castle, was not directly accessible to the attackers.
Although PRICE was not built to output scaffold structures, such structures were built internally during the assembly process.
Although the Villa Trissino was not built, it was a most influential design because it was illustrated in the Quattro libri.
Although this hotel was not built by Mr. Flagler, he folded it into his empire, and guests were pampered here from 1888 to 1932.
California provides the best evidence: although the utilities have not built a single power plant over the past decade, individuals and companies have added a whopping 6GW of non-utility micropower over that period, roughly the equivalent of the state's installed nuclear capacity.
Although Metro would not build, own or operate the project, it would provide just over a quarter of Purple Line passengers.
Although Dowling did not build on the cheap, he was not extravagant with space: his buildings had all gone straight up — real light blockers, relying on streets and low-rise neighbors for illumination and ventilation.
Although Mughals did not build extensive garden villas in Dhaka like other imperial Mughal cities in Northern India, the city could not avoid the inheriting Mughal style and influence.
