A few opinions from a guy without the experiences of the gurus here
I think barricades would only make battles longer and boring, making soldiers go and attack a barricade over and over. Wouldn't it be terribly annoying to see your Dragon set on "Fire large enemy monsters" fire and attack the barricade instead of the enemy's elephants??
Also, wouldn't that be an mindless, immobile (so not retreating) and high-hp and prot unit? vfb has suggested a summonable one, but I think that the ability to summon lots of these would be easily exploited to make battles end just by turn limit. The discussion about this has gone on fire many times before (NTJedi, are you there?
) and such a thing would bring it to the extreme.
On a final note, I'd say it doesn't seem to me that it would fit dom3's spirit a lot. The battles on dom3 have always been mostly furious battles between soldiers, mages and so on. THere are just a few, limited immobile pretenders. Starting to put in the cauldron immoblie structures on every battlefield seems just unthematic to me and distorting the nature of the game's furious battles.
Obviously just my opinions ^^