  We investigated the iron-based ladder compounds (Ba,Cs)Fe$_2$Se$_3$. Their
parent compounds, BaFe$_2$Se$_3$ and CsFe$_2$Se$_3$, have different space
groups, formal valences of Fe and magnetic structures. Electrical resistivity,
specific heat, magnetic susceptibility, X-ray diffraction and powder neutron
diffraction measurements were conducted to obtain temperature and composition
phase diagram of this system. Block magnetism observed in BaFe$_2$Se$_3$ is
drastically suppressed with Cs doping. In contrast, stripe magnetism observed
in CsFe$_2$Se$_3$ is not so fragile against Ba doping. New type of magnetic
structure appears in intermediate compositions, which is similar to stripe
magnetism of CsFe$_2$Se$_3$, but inter-ladder spin configuration is different.
Intermediate compounds show insulating behavior, nevertheless finite $T$-linear
contribution in specific heat was obtained at low temperatures.
