Arrow is right that there are two complaints here, but I would put them back together, just not the way Equinox does.
Base building in this genre has rarely been about the base itself being interesting. It is a place to put your attention during the stretch where the army is walking. Take it out and that stretch does not get better on its own, it gets empty, and the design has to find something else for you to do with those minutes. A slot system is one answer to that. It is a stricter answer than most, but it is answering the same question base building was answering, which is probably why the people who miss one are the same people annoyed by the other.
Where I am with Arrow is on walls and siege units. That is a request for a different game, and it is not the game Escalation was. Escalation ran on armies being somewhere and on you not being certain where the other side's was. Positions you crack one at a time are a perfectly good design, they just slow the map down, and that map cannot afford to be slowed down much.
The bit I would actually want nailed down is the thing Ramma mentioned, that defensive structures do not count against slots. If that holds, the turtling objection is narrower than it reads, because what HippyBait described is production getting scattered rather than defence being impossible. Scattered production is an interface problem first. I have not played the new one, so this is me reasoning from the genre rather than from the build.