Complex sentence: The Doctor uses his Sonic Screwdriver on the door and the forcefeild surrounding it burns away, they enter the mound to see an empty room with a gun called a vortex cannon in it.
Split into simpler sentences: The Doctor uses his Sonic Screwdriver on the door and the forcefield surrounding it burns away. They enter the mound to see an empty room with a gun called a vortex cannon in it.