You can close doors on both A and B sites by interacting with the switches. The doors can be broken by damaging them.
Closing doors can block enemies from entering a site. In post plant situations, closing the door can be useful for learning enemy positions.
Killjoy and Cypher are considered the best sentinels on Ascent. Their setups are usually best for B site and mid but are very strong on A site as well.
Ascent can be a heavy 5 man rush A and B map, or it can be a default map with heavy mid presence. It depends on how well the enemies are defending their sites.
Sova can shoot his Recon Bolt at B main and use an Odin to wall bang the attackers at the start of the round.