In all types of combat, jungle warfare is the most difficult, bar none.
Note that jungle warfare is the most difficult, not the most dangerous because urban and especially indoor combat are more hazardous than jungle warfare, and the casualties are higher.
But whether it's urban or indoor combat, if one side holds an absolute advantage, then terrain will not be the main factor in determining the outcome.
Firepower superiority can greatly reduce or even completely negate the advantage of terrain. A simple example: during combat, if one side holds a sturdy fortress but the other can obliterate it with artillery, not stopping at one shell but using ten, and if ten shells are not enough, then a hundred. If the artillery doesn't solve the problem, then bring in the bunker busters, thermobaric bombs, and if all else fails, nuclear weapons; anything to entirely level the fortress.
