I kind of assumed from Visser that they believed they didn't have the means to take Los Angeles directly, or even subtly, finding it easier to build a strong core of followers in one of the nearby outlying towns/small cities and working up.
Although that doesn't really make sense with the later books. But whatever.
We don't know the town they were in, but it seems like it was of significant size (major hospitals and business districts, Mariott Hotel etc). Probably big enough to provide the Yeerks with the infrastructure they needed, but small enough to comfortably infiltrate to the extent that they could build the pool etc.