Walls

Early in the game, walls are useful.  You should wall off your gold and mana vaults to make enemies work to get at your assets.

Later you realize that walls have become less useful because many of the most popular enemies fly so your walls are useless.

However, they are still good at keeping away from assets temporarily, and if you have your arrow towers inside the walls, you can do a lot of damage to the heroes perhaps even killing them before they enter.  You can also use the walls to herd heroes and non-flying troops since many follow a path of least resistance.  In doing so you can lead them to walk over bombs or towards your own waiting troops.