The only way to keep it "safe" in the way you imply is not to deploy it, i.e. you do a web service or some such.
You can't make it safe from the people executing it without making it impossible for them to execute it.
Given what you've already done decompiling would require a amount significant effort, my question would be. Why would anyone bother, as it's likely that it would require more effort than simply "rolling your own"
