to open the build menu in game you need to click the button with a hammer and saw on the bottom and if it works a menu should appear on the right side of you screen showing many tabs and buildings

now you will need to click and drag the dart monkey hall down into the place you want it to and if it worked it should begin build in you will need to wait 30 seconds for the dart monkey hall to complete and once it does there will be an extra dart monkey in your towers so he will aid you in your future land captures

thank you very much for viewing this and i hope it help you out and maybe ill put up a new tutorial on how to place down its upgrading building