could be used as a pre-placed immobile barbarian unit that can be captured, and:
have the build fortress worker ability;
the above mentioned bombard;
and require support so that a mobile unit would be "stationed" there.
been liken the city/building-type units.
just an idea-- (you could also try great wonders as units: Pyramids, Statue of Liberty, Eiffel Tower, etc. Some people might be able to use them in the mods that scaled town to single towns like SuperCiv or EFZI. Could even be used as auto-produced from wonders for cultural value) -- up to you if you want to.