No, you should kill two units in either Castle or Stronghold.
If there are at least 2 units anywhere among your castles and stronghold - kill them, if all of your castles and strongholds contain only 1 unit - kill one of them.
You are extremely lucky if there are none of your units in any castles or strongholds of yours

dancemachine, are you exactly read my post? You written about not asked.

It looks to me like he did answer the question. Why don't you ask it differently, and he can try again?

I will try to make it as clear as possible.

The card says:
" LOWEST BIDDER: Destroys 2 of his units at one of his Castles or Strongholds. If unable, he destroys 2 of his units anywhere".

Looser player have 2 units in Castle and one unit in Stronghold.

So i read card "The Horde Descends" thus that player must kill both units in Castle.

I think he can't kill one unit in Stronghold and one unit in Castle in this case.

Here you describe a situation, when you actually have 2 units in the Castle. So you just destroy them. You can only destroy 2 units in different areas, when you don't have more than one unit in ANY Castle or Stronghold of yours.