Main entrance to the dwarf halls under the Lonely Mountain of Erebor.

The Front Gate was in the South side of the mountain, at the source of the River Running which issued from it. A path also ran from it via Dale to the lookout post of Ravenhill. The hole was big enough for the dragon Smaug to enter by it when he sacked the Lonely Mountain in TA 2770.

As told in The Hobbit, Thorin Oakenshield blocked this entrance with a wall after Smaug’s death and swore to repel all claimants of the hoard within, to which he now laid claim. Yet on the evening of the Battle of Five Armies, he demolished it, tumbling it onto the besieging orcs (goblins) and leaping down to join the fray in which he so heroically died.
Encyclopedia entry originally written by cirdaneth