We've now updated the graphics for the tower build buttons so that they now have full colour and detail, rather than basic temporary placeholder buttons. They will remain grayscale until you can afford them. Once you can afford them they will fill with colour, letting you know you can now build one.
We've also updated the tower information window to be in a similar graphics style so that it is more consistent.
We have also resolved an issue with Challenge 4 whereby it was unplayable.
Enjoy!