The fastest way to grind this is horde mode. You have the chance to buy and build a lot. It will also break down over the course of the match and you can keep building more.
Just playing the single player you will earn a lot of chances to build up defenses. But it will take much longer.
I would estimate in just 3 horde matches to around wave 15 to 20, I got about 50 of the 100 defenses I needed to build.