Just finished to read manuals to understand main concepts of engine. Everything seems clear (at least theoretically), except of logic of collections, proxies, etc. Maybe, I need to spend little more time with that stuff :) Also, need a little bit more practice with Lua, never used it before.

Hey Dener,

You could use a game object attached to the mouse (set go.set_position of the object to the mouse co-ords on each update) that has a kinematic collision object attached, and then your turrets would also have a collision object (with the type and mask set appropriately so they register each other). You can then check when the mouse clicks if it's colliding with anything, and if so, send a message to the "message.other_id" of what it's colliding with (the turret), telling it to run its upgrade function, or whatever.