Hi,
For Ntier, you will need messaging(RPC) solution such as WCF to sync with your remote CE database. I am not familiar woth Citrix. However, I believe you can utilize nettcp binding with port 80 or 443 to host your WCF service and CE client can utilize it to synchronize. If you want to use Citrix Service API (if available) instead, then you would need to write your own Citrix based Service to sync.
Refer to Ntier sample for details
http://code.msdn.microsoft.com/Release/ProjectReleases.aspx?ProjectName=sync&ReleaseId=3423thanks
Jandeep
jandeepc