i made HL1 maps back in the day. quite fun. never worked out how to do trees/forests properly though. i made a quake map with a tree in it once- even after 48hrs straight of VISing (where the PC works out what can be seen from any point on the map, which lowers resource use when loaded) it still hadn't finished. had to take my tree out

tree's are easy these days, you just make a prop_static and find a tree model.

but i can't have 1000s of tree models in the map, nobody would be able to play it.