humans, square tower and round tower
 

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?

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:)