I don't know why they didn't gave spring traps any upgrades but here are some small ideas! :)
* Add a slot under the spring trap to place a small bomb. This way you will not need that much space to place all the traps. (The bomb will explode after the spring trap has been triggered.)
* The spring trap is turned into a hidden wall piece. At first it will act as a normal spring trap and slung away your enemies and after it has been triggered, a wall piece will appear in the spot it was placed. The more you upgrade it, the more hp the wall piece will get. Great way to slow down your enemies!
* The spring trap will not trigger at first when the troops walk on it. Instead the first troops will fall down in the hole that the spring trap is hidden in, unable to escape. The Spring trap will remain untriggered for one second before it springs up. This way you'll be able to slung away more enemies with one trap. The more you upgrade it, the more time it will remain untriggered.
Post your own ideas for a great spring trap upgrade! :)

