Rose can build or summon them by holding and pressing the interact button (just like the Engineer) in certain predetermined spots.
The Crystal Guardians can only be built in Herbal Assault to defend the portal (in a similar manner to how the Zombot Turrets are built to protect the Teleporter). Its attack is named Magic Crystals, and each shot deals 5 damage.
Gallery
Crystall Guardian in-game
Rose summoning a Crystal Guardian
Crystal Guardian attacking
A Crystal Guardian seen from the front
Another view of a Crystal Guardian
Trivia
Unlike all other spawnable plants, Crystal Guardians do not appear to be made of any sort of biological material, or even a plant at all.
Despite not seeming to be a plant, it still gives XP for having "vanquished potted plant" and it also counts towards the Hero to Zero achievement due it counting as a "potted plant".
This along with Boss mode and the Mushrooms are the only characters counted as plants despite not being plants
Its body shape is similar to that of the Magnifying Grass, although its sides are more bent, and is a different color.
Crystal Guardian also shoots lasers, despite the attack being named "Magic Crystals," similarly to Magnifying Grass's energy bolts.
