By tilesets I mean that you have a bunch of premade parts to make levels with, ie. 1*1m blocks that have floors and/or walls that make up the level, so that you can easily and quickly make them, then you can hand place any extra details (which you wouldnt really have any of in an RTS.)
If you’d like an example of what I mean in an RTS, then please follow this link to the game called ‘Folk Tale’ currently being developed in Unity, it will help you understand what I mean 
.youtube.com/?v=luFhbQx1rnk&list=UUa7W66RPP2X-BKPhhH1a4nw
They have of course massively updated it since with more pieces, but you get the idea. We make loads of premade pieces with small variations in height that all snap together, so that we can quickly place down anything we need, we can then use that tree blutility you made to place