pg 44. Borderers must choose from Plains, Swamp, Hills, Forests or Mountains. Zamorians get +1 to Climb, Hide, Listen, Move Silently and Spot in underground or urban conditions, so that would be helpful.
Thieves can choose a free feat at 6th level if they pick that option off thier list (last option) and could pick Fleet-footed to increase thier speed to compensate for any deductin when moving through ruins (which presumable counts as difficult ground, so each single square counts as if it were two - pg12).
The only types of terrain are desert/sandy, forest, hills, jungle, moor, mountains, plains, swamp or tundra/frozen. Geographical terrain. Urban is varried and would either fall under clear or difficult (or even full move=5ft move for exceptionally difficult terrain). However, difficult or worse terrain features can exist in any tertrain type, so it's best to assume that Urban terrain (where ruins might otherwise get categorized) is clear.
Also, Ruins could be present in any of the above terrain types (coverd by desert sands, overgrown by jungle, frozen over with ice, etc.) so where they are clear, those terrain feature penalties would apply. Otherwise they'd count as difficult or difficult +.
My approach is to give the Borderer half his bonus (rounded up), if the ruins are covered by his favoured terrain, and to delete the bonus, if the ruins are free of his favoured vegetation.
Examples: A desert ruin city recently blown partially free by a sand storm gives him half his bonus (it's half desert), a destroyed fort at a mountain pass on the other hand doesn't count as Mountain terrain in my game, but as Urban, where the Zamorian traits come in handy.