In the original game, towers were avaible in TSK 7. However, all evidence points to this being a bug/mistake. In the briefing to mission 8, you are told that (paraphrase) "in the ruins of Welheim (name of the city in mission 7), we have found plans for watchtowers, which will aid thee in thy defense." Watch tower "technology" was discovered in mission 7.
This fits nicely with gameplay, as in many games, after you learn a new "power" you are shown immediately how useful it is. Example would be I'm Zelda: You get a new item and you immediately face puzzles where the item is needed. In KaM, you learn how to make towers in TSK 8 and learn very early on how useful they are when facing a far superior force
ANYWAY, someone probably noticed this before, and thus removed them in TSK 8 for the Remake. Perhaps Litude did in the SR patches pr Lewin did when he was doing early tune-ups to the campaigns.