der_kluge
Adventurer
While there are some good answers here, you haven't really described what this creature really is; we are all guessing.
For example, what is a "tower shield", there's no such thing in 5e rules. Is the creature wearing large shields, as some kind of armour (in which case decide its equivalent e.g. scale mail), or is it literally taking cover under humungous shields (in which case maybe it has some level of cover)? Is it actively using a shield on one arm, in addition to maybe wearing some like armour?
Answer these questions, then a final AC is pretty easy to estimate.
Fair enough. The cultists referred to it as "The Wall" and my intention was that it was literally just an ogre zombie to which the cultists had fashioned about a half a dozen shields to it, such that it basically "walls" off a 10' hallway. The creature does have enough mobility with at least one arm so that it can still wield a club.