Congruence subgroup of a Bianchi group (reviewed)

Let $K$ be an imaginary quadratic field with ring of integers $\mathcal{O}_K$. For a nonzero ideal $\mathfrak{N}$ of $\mathcal{O}_K$, the principal congruence subgroup of level $\mathfrak{N}$ is $$\Gamma(\mathfrak{N}) = \{\gamma \in \textrm{PSL}_2(\mathcal{O}_K) : \gamma\equiv \pm 1 \!\! \pmod{\mathfrak{N}} \}.$$ A subgroup of $\textrm{PSL}_2(\mathcal{O}_K)$ is called a congruence subgroup if it contains a principal congruence subgroup.