Goscinny's having a go at French urban planning, architecture and labour relations. (The Asterix books got more political after the May 1968 riots, although they're still children's comics.) Julius Caesar wants to crush Asterix's village by clearing the forest and building blocks of flats. Personally, I think it would have been sharper and funnier for him to have been doing this with best intentions and to be as surprised as anyone by the ensuing catastrophe, but never mind.
Anyway, the book's first half is a never-ending cycle of failed forest clearance. If they pull a tree down, Obelix can just pick it up and put it back. If they remove it, Getafix's magic acorns can grow into a mature oak in the space of a single panel. Ultimately, the Romans only succeed because Asterix does something that backfires. He gives the slaves some magic potion, which creates a labour relations mess. (I did laugh at the slaves' negotiating tactics, though. "What if I refuse?" "We carry on hitting you.")
Our heroes then let the blocks of flats get built. Eh? Come off it. This is a self-inflicted disaster, as the Gaulish village collapses in a mess of tourist-driven price inflation. Asterix and Obelix bully one of the tenants into leaving (uh...), then replace him with Cacofonix (which is the book's funniest idea). Cacofonix is of course famously Not A Good Singer.
"The Gauls are attacking!"
"The gods are angry!"
"The building's collapsing!"
The flats eventually get trashed in a big battle that should probably have happened fifteen pages earlier.
I like the historically accurate jokes, e.g. "some of which have not fallen down". (Imperial Rome was full of cheap blocks of flats that really did often collapse.) Caesar referring to himself in the third person is also something he was prone to.
Overall, though... meh. Frankly, I'm not a fan of this one. There are people who love this book, but to me it feels like a story that keeps moving forward thanks to our heroes' bad decisions, made because the script told them to. Of all the Asterix books I own, this is the only one I don't like.