Ice Obelisk is a type of gift from brotherhood. These towers will serve as an additional protection for your town: their projectiles damage enemy's ground warriors, as well as slow down their attack and movement speed.
You can get up to three obelisks: they become available on brotherhood levels 5, 13 and 21. You need a particular amount of Cups to activate Obelisks after the end of construction, this amount depends on brotherhood level.
You can upgrade your Obelisks using Brotherhood runes which can be received from the brotherhood gift chests.
Attention: if you leave the brotherhood where you got Ice Obelisks, the towers' levels will not change, but Obelisks become deactivated. You can activate them by collecting cups in the brotherhood, which level meets requirements of towers' receiving.