random buildings on a surface

hi everybody.

first of all' i'd like to say sorry for my english. i don't know some tecnical words to explain what i'm going to say and than also for my GH skills... i've started to use it just few days ago.

I'm trying to make some buildings (boxes) on a terrain (surface)I got a lot of problems...

first of all I thought to extude all the buindings (random) and here is the first problem... using random it doesent extude all the buildings differently, but it just makes random extrusion of all biuldings toghether (i got lot of copies of the same building at different hights).

than i tryed to find a way to say to this bluindings to have their base on the surface instead of on the axis plane... but nothing... so i decide to project the curves on the surface and than extrude them... of course as a result they are no more boxes but random forms... i think i need an help.

thank you so much