hl2 or cs? cause i'm not very good with cs:s, i don't use it. In hl2 you would need to wire it so that if the tower takes a certain amount of damage, and i believe there's a field in the func_breakable and func_brush enitites that say something like that (something like damage to take, or fire on hurt) Then put the target as the monitor. Now the question is, what do you mean go off? Turn off the light? Make the screen go black? If it's the light, that's easy, the target is the light. If you mean the screen. You'll have to make sure that the texture animates, and make sure that the screen is set to an entity that toggles. maybe func_brush has it. I'm not completely sure. It'd be the kind of thing that happens when you push a button in hl1 or 2. The texture changes.