If you trap some opposing troops, the attacker still has to pay for their food. So that's cool. Also, another strategy I found is when he sends more troops in an attack to try and release those troops, you can immediately release them before they get there, upgrade your traps, and now hold even more troops hostage. At the same time, send some quicker units of your own, like the TTs, to in turn raid his land, hopefully undefended, and now you've just got the attacking dude twice.

Cool stuff for all you Gaul players.