humans, square tower and round tower

[Blizzy]

what is the difference between them?

one is higher then the other, but they both cost "about" the same

what are the advantages over each other?

[Konstantin Fomenko]

Round is slightly larger - giving your archers high attack and range value - it costs more as well. Square can fit a little bit more units on it, to allow a tower mounted trebuchet and group of archrers to fit nicely - they get a little bit less range and attack bonus and it`s cheaper.

But mostly the difference is visual