Complex sentence: Black Mesa East is directly connected to Ravenholm via a tunnel that has been barricaded off since the shelling of Ravenholm took place, to prevent zombies and headcrabs from entering the facility.
Split into simpler sentences: Black Mesa East is directly connected to Ravenholm via a tunnel. This tunnel has been barricaded off since the shelling of Ravenholm took place, to prevent zombies and headcrabs from entering the facility.