In this passage, the 1837 edition replaced the even of the earliest textual sources with ever. The subsequent 1840 edition also continued this reading, but all later editions have reverted to the original even. This 1837 change appears to be a typo rather than intentional; it was not marked by Joseph Smith when he edited 𝓟 for the 1837 edition. As discussed under Mosiah 4:7, there is some evidence that even and ever have occasionally been mixed up in the text; for cases of manuscript variation, see the list under Mosiah 2:15–16.
Seven other passages support the occurrence of even in the construction “even so that ”, although in all these examples the adjective form is the indefinite quantifier much or many:
Note that the majority of these examples are found in the book of Mosiah. On the other hand, there are no instances at all of “ever so ” in the Book of Mormon text. The critical text will maintain the original “even so high” in Mosiah 11:12.
Summary: Retain the use of even rather than ever in Mosiah 11:12 (“yea a very high tower even so high that he could stand upon the top thereof and overlook the land”); similar language is found elsewhere in the Book of Mormon text.