Civil engineering covers a wide range of topics. There are some commonalities between them, though.
1) General Math, Trigonometry and Geometry - these are used all the time in many facets of civil engineering. They are used in surveying and to calculate dimensions and quantities, among other things.
2) Design - whether you design buildings, roads, pipelines, or the like, it is important to know how to design. Part of being a good designer is learning how to best communicate your design to someone who will build it. In order to do this, you need to be able to look at something in 3D and put enough information into 2 dimensional plans for someone unfamiliar with that "3D thing" to construct it in 3D.
3) Knowledge of Materials - whether it is aggregate, concrete, asphalt, pipe, steel, or the like, you need to have an understanding of the materials used in construction and how they will work to best suit your design. You need to understand their strengths and weaknesses.
4) Statics - "Statics is the study of methods for quantifying the forces between bodies. Forces are responsible for maintaining balance and causing motion of bodies, or changes in their shape." Statics is important when designing structural elements.
5) Engineering Law - Some knowledge of engineering law is helpful to keep you out of trouble. You don't want to put your stamp on something that is outside of your purview.
6) Understanding of regulations, standards, guidelines: there are a lot of important references to know to make sure that you are following regulations, standards and guidelines.
7) Estimating - it is important to know how to estimate the cost of construction
8) Determination - this is difficult work. There is a lot of problem solving. You need to have determination to think it through.