Comment by PeterisP

The key is to have a well defined 'edge' between filled and empty areas - so that the high-valued tiles are on the corned away from the emptiness where 2s spawn; and low-value tiles are on/near the border.

Note that you can't afford to keep much unconnected duplicates - near the win point, you need to have 1024+512+256+128+64+32+16+8+4+2 which is 9/16 tiles filled already, so a few extra mid-value tiles cut your 'operating space' to near zero; so you must keep the high-values mostly ordered and agressively eliminate unneeded duplicates.