koobas.hobune.stream
Suggestions on Objects/Methods for Locked Areas of Map in Tower Defense Mobile Game
I am making a mobile game where the player "unlocks" map tiles as they pass levels (see first screenshot). 
I am looking for suggestions on how I can fill up the surrounding map, so that the tiles can gradually unlock. I would like something like what's shown in the screenshot below - every locked tile is a game object of rocks and once the player unlocks the tile the rocks disappear. The problem with this is that it creates an unnecessary number of game objects and processing, lagging on mobile.
Do you have any suggestions to fill the outside area with something like cliffs/forest/fog, which can be removed one tile at a time and is not heavy on the processing? Any thoughts would be appreciated.
