Licensing question - unityengine.dll

Hi!
We are writing client-server game, where client is written using Unity, but server is written from scratch. I want to use the same data structures as client. To avoid inventing of the wheel, can I use unityengine.dll only for using the same data structures/classes? Technically it is possible, but I`m not sure if licensing terms allow such usage.
Do I need any additional licenses?

I am not sure about the license but I wonder a bit how you want to do this technically?! Almost everything in this DLL will throw some fancy exception saying that something is not initialized correctly. If you got this to work I would appreciate to tell us about that ;), because sometimes it is really helpful if you can put some code into a UnitTest, which currently is not possible since nothing from the UnityDLL works outside a game run from Unity contexts… As far as I remember even the Vector3 structure doesn’t work…

PS: Without any guarantees, I would guess the license is no issue as long as you only use this server in conjunction with the Unity client AND you only use this server commercially if you also got a Unity PRO.