Hello Mandar,
Yes, you have to give Karnataka Common Entrance Test(KCET) with the BE students. Karnataka government gives admission in B.Arch only after getting qualified in KCET. Since, you have cleared NATA you can apply for NITs, IITs where B.Arch course is available. You can check the detailed admission process of KCET on their official website. But I'm providing a link which will minimise your work.
http://www.careers360.com/exams/kcet
You can browse with the link and check the details.
Hope this helps. All the Best.

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