Took me several tries to figure out the controls - that there are only two interactions, build and force reload. At first I thought arrows pointing up meant upgrade.
I couldn't place and load towers quickly enough when enemies appeared to kill them, so best strategy still remained building at the center of the map. Also quick reloading towers near center allowed me to never run out of bullets in them, however that being a little bit tedious.Towers could also be better at indicating if they're going to consume resource or if it going to pass though.