In Dungeon Defenders why can't I summon a tower even if I have the mana?

It's possible that you are out of Defense Units.

Every defense (trap, aura, tower) costs a certain amount of Defense Units to place down. You can see this when inspecting a defense, as well as in the radial menu when inspecting your hero defenses.

In the top right corner of your screen should be a number xx/yy, where xx is the current amount of defense units that you've used and yy is the total number of defenses allowed for the level that you're on. If you are out of defense units, or don't have enough for the defense you wish to place, the game will not allow you to put one down.

There area few things that could attribute to this:

• Do you have enough Defence Units left to place down that tower?

• In regards to the monk auras, 2 auras of the same name cannot overlap or touch.

• There is also a restriction on how close to the crystal or spawn point the defence can be, you can never have an aura or a trap overlap a crystal for example.