Number theory is not obvious at first but certainly at university level is not one of the hardest areas of maths. The more advanced you research into any area you will come up against difficulties, but in my experience the majority of undergrads find the pure math component relating to number theory entirely reasonable. In fact I am teaching some of the basics of it relating to prime number theorems and modular math to some year 10's at school and they are picking it up easily. I m not saying its simple to pick up a pen and solve fermats last theorem but there are many that can easily be learnt and understood.
In my view some of the harder parts of mathematics include advanced PDEs in the context of applied maths but as they are interesting to many, this maintains the focus!