So, the unit that I don't recall ever seeing in Civ is a 'Special Forces' unit like Navy Seals, Army Rangers, etc. Yes, there have been spies, but these would be different and I would see these units as being very expensive to build and require a few different city buildings and/or civ advances. Their use could be for things like taking out a fully fortified SAM site in a mountain range, or taking out enemy units like tanks, etc. They could also be used to get behind enemy lines without the enemy knowing it and help pave the way for a preemptive strike. What do you think?