Spies are huge, both in Sunzi and in RON. They're easily intercepted by scouts and watchtowers, but they can give you good operational intelligence of the enemy.

The shroud makes it less useful to infiltrate the enemy's actual units, but if you can find their military production centers, planting informers there is very useful. You can see what units they have produced, and can tailor your own production to effectively counter their units. Assigning the map camera view to one of your memory locations is very helpful - you can just hit the hotkey to center on those military buildings, and click on them directly to receive a full Infiltrator's report.

In actual open combat, a spy is likely best used to target enemy heavy units, rather than small infantry or even cavalry units. Siege units have a dramatic effect on your cities' survivability, so those are usually my main target with spies. Beware of enemy scouts, which instantly reveal nearby unfriendly spies (one of the game's rules weaknesses, in my opinion - counter espionage should be a skill that requires Craft to activate, revealing spies within range for a limited duration only). If you have a small defense force, consider engaging the scout at range to knock him out, then withdraw your forces and bribe the siege with your spy.

Formations are a flexible way to enhance your fighting effectiveness. I saw a replay video where an invader ran his army single file around the visibility radius of the defender's city in order to launch an attack undetected against the capital. The attack forced the defender to split his attention and resources across two fronts (because another invader was attacking the more predictable border village).

Misdirection, intelligence, and placing your opponent at an operational disadvantage - all lessons from Sunzi's Art of War. No single game can use all the stratagems, but I think RON does a good job of incorporating some.