Turret placement is a game of precise geometry, aggro manipulation, and predicting the enemy's pathfinding AI.

Novice players tend to panic and place their defensive buildings directly in front of the incoming threat, often right at the bridge.
The Anti-Tank Center Pull
The absolute most important placement to memorize is the center pull, designed specifically to deal with massive, building-targeting tanks.

As the tank trudges toward the center, it will be simultaneously shot by your defensive building AND both of your Princess towers.
Wait until the tank crosses the bridge before placing it.A mortar cannot hit units directly next to it.Vary your placement slightly if they have heavy spells. Advanced Placement Tactics
Your goal is to use your building as a physical meat shield, absorbing the artillery fire while your troops cross the bridge to destroy it.

Place a splash-damage building like a Bomb Tower directly in the center to safely absorb and destroy the endless waves of cheap units.
Attacking UnitHow to Alter Your GeometryAir AssaultMust be placed slightly higher and more central, as flying units ignore the river and take a direct diagonal pathUnits with Spells (e.g. If you are you looking for more info about tower rush look into the site. , Giant + Lightning)Place the building extremely high up by the river so the lightning spell cannot physically hit the building and your tower simultaneously Geometrical Dominance
When you master turret placement, your opponent will feel as though their units are fighting through thick mud.

Practice your center pulls until you can do them with your eyes closed.