Use event and delegate in subclass

Events may only be invoked from the class that declares them.

From outside of the definition of a class (even in a derived class) you can only register and unregister from an event. Inside of the class, the compiler only allows you to raise the event. This is a by-design behavior of C# (which actually changes slightly in C#4 - Chris Burrows describes the changes on his blog).

What you want to do here is provide a RaiseLogEvent() method in the base class, which would allow the derived class to invoke this event.

public abstract class Base
{ 
  public delegate void logEvent(String message, int level); 

  public event logEvent log; 

  protected void RaiseLogEvent( string msg, int level )
  {
      // note the idomatic use of the copy/test/invoke pattern...
      logEvent evt = log;
      if( evt != null )
      {
          evt( msg, level );
      }
  }
} 

As an aside, you should consider using the EventHandler<> delegate type, rather than creating your own event types when possible.

Because events can only be called from the declaring class. Just create a method in the base class to call it:

protected virtual RaiseLogEvent(string s, int i)
{
  log(s, i);
}

So you can use it in deriving classes, and even override it.

On another note, I would strongly advise you to follow the design guidelines for events, and create an own EventArgs class, and use the EventHandler<T> delegate.