Strong faction

In RTW my favorite tactic was to get early a settlement of one of the strong factions (rome or egypt), get walls up and watch them bleed themselfes dry. I love playing defensive sieges.

Anyway, in BI I find 2 problems: Only 3 factions have "good" walls: The two romans and the sassanides.
But what`s worse: I find no really "powerful" enemy. The only strong oney are the romans again, but with enemies on every side and torn by revolutions, I allways pity them. So on whom shall I pick?

Playing one of the Roman factions seems the obvious choice, since their campaign goals are based on turtling and playing defensively, with just a little territorial expansion required to capture the other faction's capital and unite the empire. You can withdraw (if necessary) to a core set of provinces, consolodate your economy, then sit tight for a while as the other factions pound on each other. Then if you can hold things together, make a final push into the rival Roman faction after they're weakened enough.

As the Sassanids you'll have to be more aggressive with expansion, since the campaign goals include taking Alexandria and Constantinople. You'll have to expand anyway to get any kind of economy going. You'll get some cities with big walls as you defeat the ERE, and you might eventually be dealing with barbarians going against your larger cities on the northern frontier. So maybe you'll have a few good defensive sieges along the way. I think the main choice would be ERE or WRE though, if you mainly want a defensive, turtling campaign.