So ...

[Bob the Insane]

I don't think Heavy Cavalry is supposed to be able to swim, so it probably wan not exhaustion...

I will have to give this a go... Wait until a unit is swimming and then send a non-swimming unit to attack them and see what occurs..

Scout: The enemy are swimming the river sir!!
Commander: ah-ha... Bob, take your cavalry down there and kill them all...
Bob: umm... errr... I don't think...
Commander: I don't pay you to think, get going...
Bob: But I don't think I can swim in all this armour!!
Commander: Are you refusing a direct order!?!? I will have your head!!!
Bob: Ah.. well... of course we will attack immediately!!

Bob: Right lads, our target is the men in the water, form line and CHARGE!!!

Splash... gurgle... bubble...

Swimming Barbarian: Well that was odd!!!

[Kraxis]

I'm getting more and more into the 'thinking' of the WRE.

I have noticed in my own several campaigns (mostly tests and two serious ERE campaigns) that the WRE loves to attack Campus Frisii. In every single case they have attacked and taken it. I have also noted that several others have indicated that the WRE has taken that city in their games, and now froggy experiences an attack when she takes it early.

Campus Frisii = WRE attack!