Thanks for playing and the feedback.
You need to use a longer ranged tower, such as the starting catapults, to deal with the red guys. Putting some of them near other towers, especially the shorter ranged ones, can help. The catapults also get range upgrades, which help to deal with the threat from further away.
We did acknowledge them attacking from off screen and tried adjusting the map so you couldn't put your towers too close, but they have long enough range it can still happen. If we develop further, we will rework the map and their range to try to force them onto the screen before they start to fire.
I wanted to highlight the related resources when a tower is selected, and show the number in red when lacking, but ran out of time.