  We prove equidistribution theorems for a family of holomorphic Siegel cusp
forms of general degree in the level aspect. Our main contribution is to
estimate unipotent contributions for general degree in the geometric side of
Arthur's invariant trace formula in terms of Shintani zeta functions. Several
applications including the vertical Sato-Tate theorem and low-lying zeros for
standard $L$-functions of holomorphic Siegel cusp forms are discussed. We also
show that the "non-genuine forms" which come from non-trivial endoscopic
contributions by Langlands functoriality classified by Arthur are negligible.
