  EuSn$_2$As$_2$ is a trigonal A-type antiferromagnetic topological insulator
with the moments aligned in the $ab$ plane and with a N\'eel temperature
$T_{\rm N} = 23.5$ K. Here we report that an EuSn$_2$As$_2$ crystal exhibits a
broad peak at $H_{\rm c1} = 1100$ Oe in the field derivative $dM_{ab}/dH$ of
the $ab$-plane magnetization $M_{ab}(H)$ at temperature $T=2$ K, demonstrating
the presence of trigonal antiferromagnetic domains. We model these
$M_{ab}(H,\,T=2\,{\rm K})$ data and obtain the trigonal anisotropy-energy
coefficient $K_3$ that is 13.3 and 3.7 times larger than those we previously
reported for single crystals of the trigonal compounds EuMg$_2$Sb$_2$ and
EuMg$_2$Bi$_2$, respectively.
