Qanṭarah is an arched bridge or vaulted structure; tuktanafan(n)a involves enclosing it on both sides; and qarmad refers to brick or similar hard masonry. Tushāda conveys raising or constructing solidly.
The emphatic heavy nūn in tuktanafan(n)a adds solemn insistence following aqsama, “he swore.” The grammatical emphasis matches the image of determined, complete construction.