You will definitely need to add some changing cursor (pointer), which would communicate what the current 'mode' of it is, and whether it is above a valid clickable location or not; consider making a 'ghost outline' of the different tiles and when the cursor is over a valid buildable area, display such outline (perhaps with a text over it saying "Click to add monster trail" and "Click to add Defender area" or some such); similarly, if I click "Water tower" or "Main tower", then I should be able to see what area it will protect/attack if placed where the mouse is.
It's kinda hard to tell at the moment, because the issues I've pointed out make it difficult to play your game; consequently, a player then spends most of their effort "fighting with the controls", and not enough focusing on the actual gameplay.
Perhaps once you have an update, it will be possible to inspect the gameplay.