So this is what I had to do to get a composite key to work

ScrewturnPageKeyword.cs

public class ScrewturnPageKeyword
    {
        private int? _hashCode;

        public virtual string Page { get; set; }
        public virtual string Namespace { get; set; }
        public virtual int Revision { get; set; }
        public virtual string Keyword { get; set; }

        public override bool Equals(object obj)
        {
            if (obj == null) return false;
            if (this == obj) return true;

            var pk = obj as ScrewturnPageKeyword;
            return (this.Page == pk.Page
                && this.Namespace == pk.Namespace
                && this.Keyword == pk.Keyword);

        }

        public override int GetHashCode()
        {
            if (!_hashCode.HasValue)
                _hashCode = base.GetHashCode();

            return _hashCode.Value;

        }
    }

public class PageKeywordMappings : ClassMap<ScrewturnPageKeyword>
    {
        public PageKeywordMappings()
        {
            Table("PageKeyword");
            CompositeId()
                .KeyProperty(m => m.Page)
                .KeyProperty(m => m.Keyword)
                .KeyProperty(m => m.Namespace);
            Map(m => m.Revision);
        }
    }