Try this first; Save this map to a different name and use that new map for experimenting. With the new map draw a box around no mans land, where you plan on making changes,and click on select inside. When it's all selected save it to a prefab, if you wish. Then delete it and replace it with the terrain that is less dense, don't bother manipulating it just yet, just put it in there and try and compile. If it still doesn't compile go to a lower number and keep trying it until you have something that will compile. Then manipulate the terrain the way you like. Just a thought.

Okay, will do, thanks.

You can remove density

Like this select vert like in pic,hit Ctrl+Shift+Q








to add a row do same as pic 1 and hit Ctrl+Shift+A

Hope its clear for you not so good in explaining stuff in English

me Dutch

Your explanation was fine.