Actually, as Alaska Dave also pointed out with the quotations from Wiki, which I left out in my initial post, but did read, a tower is “A tower is a tall structure, usually taller than it is wide, often by a significant margin”.
I also note the openstreetmap definition: man_made tower: A tower is a building, which is multiple times higher than its diameter
And that is the reason why I think it is not appropriate for stupa. There are many forms of stupa, and many do not classify as tower.
Just to give some ideas:
Borobudur is a terraced flat stupa with many small stupas on it http://www.essential-humanities.net/world-art/south-asian/
Here we have a small stupa: http://commons.wikimedia.org/wiki/File:Small_Stupa_at_Kathesimbhu_Stupa_(5186682593).jpg
Many stupas are small, i.e. for example in Wat Mahathat has many small stupas (Chedis) on its ground, in fact also modern Wats have many little commemorative Chedis on their grounds (platforms in the foreground https://ssl.panoramio.com/photo/85378416))
There are basically three stupa types: dome, terrace (Borobudur), and tower-like
(http://books.google.se/books?id=nzqK8dDCM0UC&pg=PA221&lpg=PA221&dq=are+stupas+towers&source=bl&ots=1xkBEUG7Wr&sig=cMvhnPnpemo70Osoc0HZ6c0UsPo&hl=en&sa=X&ei=npe5Utj5K-T34QTbk4Eo&redir_esc=y#v=onepage&q=are%20stupas%20towers&f=false).
The Chedis are basically dome shaped derivatives, with terraces as base, while the old Mon style quadratic tiers of the chedi of Wat Chedi Liang (Wiang Kum Kam) are terrace based.
The tower-like stupas are actually the Pagodas of the Far East. (note: pagoda should also be considered in this discussion, it seems not defined so far, and used in OpenSeaMap in a somewhat inappropriate definition (temple), also a few other tags refer to Pagoda http://tagwatch.stoecker.eu/Asia/En/tagstats_building_pagoda.html )
My point is simply to have a general way of tagging all stupas (whether tower or not) in the same way. A further question would be whether to include Pagodas in this, or treat them separately. While it’s easy to discern the multi-tier pagodas in China/Japan, the Shwedagon Pagoda in Yangon looks like the normal spired Stupa/Chedi, and is officially also called Shwedagon Zedi Daw (note “Zedi”) - I guess the name Pagoda is a misnomer from colonial times.
Can multiple tags stand?
I admit, I am not familiar enough how appropriate it is it double tag the same thing several times. i.e. is it ok to tag some stupas with the tag tower, as well as a second time with another tag (e.g., monument:stupa), which refers to all stupas and chedis? It seems like double work and would lead to conflicts in rendering (which one is valid, or both are shown next to each other, information overload)
Sitenote:
I do notice that conventions for towers are rather confused, i.e. multiple ways to define the same thing, e.g.
man_made campanile (bell tower italy)
man_made tower - tower:type=bell_tower.
as mentioned, getting consensus in a world-wide project seems hopeless…
Perhaps at this point, stupas don’t need to marked at all, since they are usually part of a place of worship.
Perhaps one should discuss how to have a consistent nomenclature to label all historic sites, including Ayutthaya, Sukothai, Kamphaeng Phet, Si Satchanalai, all the Khmer sites (Pimai, Phanom Rung, Preah Vihear), etc.,. If then subfeatures within a site are to be labelled, can then be taken on afterwards.