Government & Defence Entrance Exam

Hello,
Eligibility for IAS is passed graduation in any discipline from a recognised university, there's no minimum aggregate required in this case, you need to be 21 years old and you can apply even in the final year. Now, coming to your question UPSC is more humanities oriented and people from humanities background have an added advantage they're already well acquainted with the subjects included in the syllabus, for Science and other streams they do not study humanities subject at 10+2 and graduation level. But, this in no way mean that people from other disciplines cannot qualify this exam, if we see the previous year trends those from engineering, medical background and alike have qualified UPSC, in fact this is also a very reason that UPSC doesn't discriminate among aspirants and hence allow people from diverse background to appear for the exam, so there's no reason why you shouldn't with B.Arch Degree
Eventually, what matters is your preparation and strategy, UPSC takes two year; one year for preparation and another one for appearing for three stages namely Preliminary, Mains and Interview. This exam tests perseverance like no other exam, and if you're determined and focused, you'll definitely qualify this.