Harder: A delphi TCP server. Accessed from clients using IdTCPClient. Requires a Windows server where you can run your own executables.
Argh Why on earth should it require a windows server ?

There is no problem in doing a cross platform TCP server. I did once some times ago for a MUD i was designing. That's exacly what we need so i could probably copy some code from there.
This evening i'll search it.
A TCP server is way more performant and easy to develop then a web service IMHO.

About the server, we can initially host it by ourself (on my PC for example), then we could find an host when we have something completed (the same way you would do with a MUD).