At a guess, and knowing a little about history, I suspect that the full text runs something like this:
"I put [(someone's) head] on top of the gate... [such is how all traitors are served.]"
roflmao... funny. I was thinking exactly the same thing.
More likely this:
"[Dear Milkman, I put your payment in the mailbox] on top of the gate"
If the inscription comes from the beginning of Darius' reign, then he probably did put a head on a gate. He fought a two-year civil war (522-520 B.C.) to keep his throne, defeating nine enemies in nineteen separate battles, according to the famous Behistun inscription. The predecessor he overthrew, Smerdis, was very popular because he had decreed that no taxes would be collected for three years. And today's politicians have the nerve to claim that George W. Bush's tax breaks will break the bank.
How's that for a tie-in with modern politics?