The construction looks fairly old, but by an order of years, not centuries. If you had to guess, you'd say fifty years old, maybe. The mortar is solid and dry, but not particularly strong; it looks to be a mixture of impure sand, or maybe just mud, and lime.
On closer inspection, Zangua's comment seems to be correct: the rounded objects have the look and feel of bone. They are all oriented in the same direction, and it doesn't take long to conclude that the seal is made of goblin skulls, facing away from this chamber, into the tunnel they're blocking off.