ok here's one solution might not be the best but try it
also it depends do you want to make different heights of the fills or you want the whole city same level,
first you probably got masterplan on DWG or DXF format so
if you have 3d MAX it is easy
1). open masterplan in AC
2). select the desired files/polilynes that ieg. will have height of 15 m and export them as dwg or dxf,
3). in 3d max import the file and select all and then apply EXTRUDE modifier and set the desired (height)/amaunt (in max) of extrusion
4). export the model as .3ds
5). and in AC 9 (if you dont have AC 9 then find the import 3ds add-on for older AC) import/convert the 3ds created in max earlier and plase the "buildings" as .gsm objects in their place,
6). repeat these steps for multiple height values
hope this helps, bit complicated for reading but piece of cake to do it
ps. you have to be careful with the unit conversions all the time
giza
Gezim Radoniqi | Architect | BIM Manager @ 4MGroup
ArchiCAD user since version 6
AMD Ryzen 3950X CPU, 64 GB RAM, NVidia RTX 3060 12GB