To find a way inside the castle in level 3 of LEGO Marvel Super Heroes 2 go over to the cage to the left of the gate. Switch to Doctor Strange and use time controls to fast forward the time until the cage is broken on the ground:
Switch to Groot, make Groot get large and enter the cage. Switch back to Doctor Strange and wind time backwards until the cage rises high enough. Switch back to Groot, exit the cage and open the gate by spinning Groot forwards.