Like most people, my terminal servers are constantly being probed via brute force attacks trying to find a weak spot. The better answer is to put the terminal server behind a VPN. Short of that, I like setting up Duo Security for two factor authentication. Another alternative (and perhaps in addition to Duo) is to detect and protect against brute force attacks on your firewall.
Here's how I configure that on my Fortigate firewall.
First, enable the Intrusion Prevention module (if not already done) in Config -> Features
First, I enable the IPS rule for RDP brite force attacks. I set a threshold of 15 over 900 seconds (15 minutes) with a block duration of 259200 seconds (3 days).
Then you go to your RDP policy and set the default policy for your RDP policy.
That's all you need to do. If you want to see what IP addresses have been blocked, go to Log & Report -> Security Log -> Intrusion Protection