MARVEL is an untethered quadrupedal climbing robot capable of agile locomotion across floors, vertical walls, and ceilings in ferromagnetic environments. It achieves state-of-the-art climbing speeds—0.7 m/s (2.12 body lengths/s) on vertical walls and 0.5 m/s (1.51 body lengths/s) on ceilings. MARVEL can step over 10 cm gaps, overcome 5 cm obstacles, and transition seamlessly between different surface orientations. Its key features include electropermanent magnetic feet, magnetorheological elastomers, torque-controlled actuators, and a model predictive control framework.