The thing is: The layout of Hightower makes it very fun to play on, jump around on, do random stuff on (at least for me.) Most classes can do the thing that creates the most fun for them - Demos, Soldiers and scouts can (explosive) jump all day long, Pyros can airblast people down the cliff, spies can be obvious and facestab people (or use the many lesser-used roads and actually be sneaky), Heavies can... shoot at things and sit at the cart (that's what heavies do, right?) and Engis have a whole room dedicated to getting spammed out by Demomen. Even snipers have a balcony where they can shoot exclusively at other snipers while not getting any other sightlines before they get marketgardened.
All that combined in a compact, easy-to-remember layout where everyone knows where to go after like 5 minutes creates fun gameplay.
On the other hand, the objectives themselves are "almost impossible" to push, because you have two very chokey paths right at the spawns (ramp and elevator) that completely reset your progress if you don't manage to push through, which usually takes coordinated effort, which is something that pubs are notoriously bad at. That makes me say that from that perspective, the map is kinda bad.
Ideally, you would create both fun gameplay and a fun-to-fulfill objective, but as the success of Hightower as a "casual" map shows, it sometimes is enough to excel in one category (or maybe 2, i guess noone can really say that the map looks bad.)
When you try to take Hightower as an inspiration (and i did exactly that when planning out plr_hierarch,) look at what creates the fun gameplay that many players can't get enough off, and actively ignore or at least try to not do what they did with the objective. That's my approach, and we'll see how this works out.