For the first 2 parts of the build, you need 300 mana. Place 2 spike blockades on each of the choke points leading to the crystal. Then place 1 bouncer blockade in front of each duo of spike blockade. Do this gradually over the course of the first 3 waves. Upgrade these defences once, so that's about 900 mana total to upgrade, so far.
For the Third part of the build, you need to(if you can,) place 2 harpoon turrets behind each set of blockades. Upgrading these will set you back about 600 mana. But it is still worth it, preparing for the ogre at the end of the level, last wave.