Collocations for terrain - noun

flat
steep
hilly, mountainous, rocky, rough, rugged, uneven
arid, barren, desert
boggy, marshy, swampy (esp. AmE)
difficult, harsh, inhospitable, treacherous
difficult terrain for walking

familiar, unknown
open
Tank warfare dominated campaigns in open terrain.

urban

cross, navigate, negotiate, traverse
explore
know
local fighters who know the terrain

across … terrain, over … terrain
It took us the whole day to trek across the rocky terrain.