Seem to have found a pretty foolproof way of kicking ass

So I'm playing with the Third Age and Divide and Conquer mods on Very Hard/Very Hard, and seem to have found a way of getting rich and destroying enemies pretty quickly! I've done this with both Isengard and Mordor so far and will keep on trying it out...

Basically I marched a full army of my best soldiers to a huge, but pretty undefended, city in their lands, while being neutral with them so not being attacked on the way. I snuck a spy in for ease and attacked as quickly as I could and captured the castle (Isengard and the Black Gate, respectively). Then, as quickly as possible, I attacked another, smaller nearby town which was also fairly undefended.

Both of these battles left my armies in pretty poor condition a long way from home, with enemy armies all around so - I destroyed all of the buildings in the smaller settlement (for a LOT of cash), then asked the enemies for a ceasefire in exchange for their smaller settlement back, which they were very happy to accept. This gave me a huge amount of money from the destruction of the buildings, it means that they have to spend a massive amount rebuilding their smaller settlement, gave me a well-fortified castle in their territory and means I'm not at war with them, so I'm able to spend some time rebuilding my armies to defend it in case they attack again.

I did this quite a number of turns ago with Mordor and haven't got any repercussions yet - just slowly increasing my army and defenses at the Black Gate - and have just done it again with Isengard.