This research presents a robust state estimation framework for legged robots, grounded in the theory of the Invariant Extended Kalman Filter (InEKF) on the \( SE_{2+2N}(3) \) Lie group. By explicitly modeling multiple contact points for each leg, the filter accurately captures complex ground interaction dynamics during locomotion. The system's propagation, driven by IMU kinematics, is tightly coupled with a multi-modal correction phase that integrates visual odometry, joint encoders, and pressure sensor arrays. This strategic fusion of proprioceptive and exteroceptive sensors significantly enhances system observability, effectively mitigating drift even in dynamic environments. Consequently, the proposed architecture delivers precise and consistent state estimates essential for stable legged robot control.