This is a former gunpowder proving ground at the feet of A Kung Ngam Shan. On most online maps, it was marked as a Gunpowder Factory (which is not convincing looking at the small rounded structure) or a Gunpowder Storage (also not convincing), but it should not be used in this way, as the structure is round and lacks a roof even when the wall is well maintained. Upon visit, it was marked as a Gunpowder Proving Yard, used to test gunpowder supplies to ensure that they could be used properly in combat. The cone-shaped structure of bricks keeps it unharmed after facing shock waves from small explosions to test gunpowder in the structure. Now the place contains monuments that are built in memory of soldiers who sacrificed themselves in the battle of Hong Kong, mostly in the combats which took place on Hong Kong Island.
P. S. You may notice that the mark on Google Maps shows "Former Gun Powder Proving Ground". I edited that.