My hope is this will get fixed soon.  This is a really cheap strategy for the early game, I wouldn't suggest it unless you want to make life easier.  It works on all difficulties, though higher difficulties you have less time to do it.

Step 1: Explore

Step 2: Find AI and check city for defense

Step 3: If only spouse take the city, just walk in and take the city (or cities).  If defended go back to step 1.

Step 4: Leave city and auto-resolve against spouse (tactical battle will cause spouse to run) you will always win because the spouse has no attack and it's good xp.

Step 5: Return to step 1.

Step 6: ????

Step 7: Profit!!

How long it takes the AI to get defense units varies with the difficulty and the particular AI, but even on ridiculous I can usually do this to 2-3 AIs if I want to.  Janusk and Sovereign is all you need, though more champions makes steps 1 and 2 easier and so increases the odds of finding the AI before they defend their cities.

The reason this works so well is that once you take the city the AI will run off and cry in a corner and start over.  You do NOT need to actually defend the cities you just took, or any of your cities really.  Once I had an AI take it's city back, but then it's sovereign wandered off and I took the city back.  99% of the time there is absolutely no risk and significant reward for this.  Especially since 1.08 it seems (at least on ridiculous) the AIs get 5+ resource nodes around their start location, making for some really awesome cities.  Sometimes with 3 gold, tech, or arcane resources and a shard or two on top of the food production.

My suggestion for an easy solution is give the spouse a weapon and make them actually defend the city, with the 30 health they have now an actual army would be required to take an AIs starter city.  Or make the AI take any/all undefended cities they are close to when at war with them.  Or better yet, both.