  We study the asymptotic and qualitative properties of least energy radial
sign-changing solutions to fractional semilinear elliptic problems of the form
\[ \begin{cases} (-\Delta)^s u = |u|^{2^*_s-2-\varepsilon}u &\text{in } B_R, \\
u = 0 &\text{in }\mathbb{R}^n \setminus B_R, \end{cases} \] where $s \in
(0,1)$, $(-\Delta)^s$ is the s-Laplacian, $B_R$ is a ball of $\mathbb{R}^n$,
$2^*_s := \frac{2n}{n-2s}$ is the critical Sobolev exponent and $\varepsilon>0$
is a small parameter. We prove that such solutions have the limit profile of a
"tower of bubbles", as $ \varepsilon \to 0^+$, i.e. the positive and negative
parts concentrate at the same point with different concentration speeds.
Moreover, we provide information about the nodal set of these solutions.
