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Glassy dynamics in a confluent monolayer is indispensable in morphogenesis, wound healing, bronchial asthma, and many others; a detailed theoretical framework for such a system is, therefore, important. Vertex model (VM) simulations have provided crucial insights into the dynamics of such systems, but their nonequilibrium nature makes it difficult for theoretical development. Cellular Potts model (CPM) of confluent monolayer provides an alternative model for such systems with a well-defined equilibrium limit. We combine numerical simulations of CPM and an analytical study based on one of the most successful theories of equilibrium glass, the random first order transition theory, and develop a comprehensive theoretical framework for a confluent glassy system. We find that the glassy dynamics within CPM is qualitatively similar to that in VM. Our study elucidates the crucial role of geometric constraints in bringing about two distinct regimes in the dynamics, as the target perimeter $P_0$ is varied. The unusual sub-Arrhenius relaxation results from the distinctive interaction potential arising from the perimeter constraint in such systems. Fragility of the system decreases with increasing $P_0$ in the low-$P_0$ regime, whereas the dynamics is independent of $P_0$ in the other regime. The rigidity transition, found in VM, is absent within CPM; this difference seems to come from the nonequilibrium nature of the former. We show that CPM captures the basic phenomenology of glassy dynamics in a confluent biological system via comparison of our numerical results with existing experiments on different systems.