Spoiler Alert, click show to read:
The only trouble is that I would get a clear heads-up about the invasion the instant I catch a spy in my territory. I would be well-prepared for the invasion if I see a spy or catch one, and the enemy attacks 12 turns later. The AI should once in a while attack blindly (If your relations with them are bad). It wasn't always the case that they knew what to expect anyhow. But they should come prepared for anything, no matter how big my empire.
There should be a feature to assess the enemy strength when maps are exchanged, making you a bit more reluctant to share your maps with the enemy. Once you show them your maps, that faction should be able to assess your strength and plan accordingly.
Alternatively, princesses, merchants, assassins, priests and diplomats should be able to at least relay how many units there are in enemy territory, if not tell the AI everything about them. I use diplomats as spies and merchants as well.
Merchants cost nothing per turn, and function like a poor-man's spy. Offering trade rights and map exchange should come with the penalty of having your borders open to such riff-raff (as it does now) and having your military strength revealed to the enemy (as it should).
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