Slitherine

What happens if you can't place a compulsory terrain piece on the board ?

I know it sounds impausible, but checking the rules I was wondering what would happen in the following circumstance

1) You place a river/coastline
2) Roll a 5 & place a 12" steep hill with a village in your opponents half of the field.
3) Roll a 5 & place a 16" steep hill compulsory on your opponents half of the field.

Your opponent then rolls a 6 and has no room to deploy his terrain.

I believe it would be discarded. The compulsory terrain pieces are compulsory in the sense that they must be selected. There is no rule that I am aware of to say that they must get on the table.