Thanks a lot for the links, but some of the resources are no longer avaiable (well those cna wait for now, i'm still getting the hang of the GUI) at the moment I've tried to get my first custom 'tower' working, but there are some problems around it.
Basically I want to create a Castle that fires multiple arrows from different positions, different firing times to create an arrow shower.
So, at the moment, I created a dummy ability for the upgrade icon on the Castle, a dummy unit that can be trained (renamed as an upgrade) for extra attacks, dummy archers that can attack created on the Castle (small x,y variations).
I'm using a simple trigger that tracks the creation of the dummy upgrade unit to apply the updated icon label, and to remove availability of the unit once it reaches cap level. Up till here the upgrade works fine, but I can't destroy the dummy archers if the Castle dies - I looked up the pick units, and didn't work with units having locust, and the only way I've managed to destroy the dummy archers is with a unit group variable, but its global so if I use that i can only have 1 castle.
How do I change the trigger to be able to record every single instance of a Castle and efefctively preserve the destroy trigger? (I'm still working on GUI, obviously)
Also, is there a setting to make projectiles inaccurate? I think I havent noticed any of those in the object editor.