I have a degree in Civil Engineering and I want to continue my studies in the field of mathematics. I think the only option that I have to enhance my application is to take the GRE mathematics subject test. If I can get a good score, will I have a chance to get an admission in a top university? Thanks.
I'm going to guess that it will be difficult, depending on what you mean by a "top university". Your CE background has some math, it is true, but you are probably missing quite a few of the courses that math graduates will have had.
The math subject test itself is very difficult and tries to cover many topics. Even a math graduate will have had no courses at all that cover many of those topics and even they won't be able to answer them. I did quite well on the test (with a very strong math major) but still felt terrible on finishing the exam.
The only way to be certain, of course, is to make application to a few places. But I think you might need more math background than would be typical in any engineering degree. An applied math program might be within reach, though.
There is a lot of competition and most of it coming from people with a full math curriculum behind them.
I'd suggest, however, that you find a way to talk to (or even visit) a potential institution and discuss it with an admissions person. They might have specific advice about what you need and how you could get it. A pretty steep climb overall, I'd think, unless your background is unusual.