In this study, solidification process inside a water filled rectangular cavity is numerically investigated. The mathematical model is validated by comparing the current numerical predictions with the available analytical, numerical, and experimental results for three different test cases: one-dimensional conduction dominated solidification, natural convection in rectangular cavity, and natural convection dominated solidification in rectangular cavity. For all three cases, some good agreements are achieved in terms of isotherms, interface positions, and streamlines. After validation, time-wise ice formations are represented, and comparisons are made between bare and finned wall cases. In addition to these, further analyses are carried out by neglecting the buoyancy forces to introduce the differences between natural convection dominated and conduction dominated models. The results emphasize that natural convection has a critical effect in actual phase change processes.