The flow structures, drag coefficients ($C_d$) and vortex shedding characteristics around a single square cylinder and twin side-by-side square cylinders were experimentally investigated with various Reynolds numbers (Re) and gap ratios ($g^*$) in a vertical water tunnel. The Reynolds number (Re) and gap ratio ($g^*$) were 178 < Re < 892 and $0{leq}g^*{leq}2.5$, respectively. The flow patterns and vortex shedding frequency were determined using the particle tracking flow visualization (PTFV). The flow structures, velocity properties, and drag coefficients were calculated using the particle image velocimetry (PIV). The topological flow patterns of vortex evolution processes were plotted and analyzed based on critical point theory. Furthermore, the flow structures behind twin side-by-side square cylinders were classified into three modes single vortex-street mode, gap-flow mode and couple vortex-streets mode. The maximum $C_d$ occurred in the single vortex-street mode, and the minimum $C_d$ occurred in the gap-flow mode. The highest Strouhal number (St) occurred in the single vortex-street mode, and the lowest St occurred in the gap-flow mode.