Some cool ideas here, and a coherent effective art style + atmosphere. Learning how to set up your first runes and towers and work out what they do or how effective they are takes a long time though, and this sort of "lots to learn" is a problem of many tower defense games already!
Perhaps some single rune attack towers that start the player off, and reserve the more complex arrangements for more and more powerful structures would give the player more time to learn the systems.
Good job!