Episode 11 - Believed to be called 'Journey To The Centre Of The TARDIS.
This story is written by Stephen Thompson, who wrote The Curse of the Black Spot, and directed by, newcomer to the show, Mat King. It stars Ashley Wasters. In DWM 454 Thompson revealed it will feature "multi-dimensional geometry" but "the star won't necessarily be the usual person. You might not even see the star."
Episode 12 - Written by Mark Gatiss and stars Dame Diana Rigg and her daughter Rachael Stirling. It is directed by Saul Metzstein. Like Episode 9, it's a bit scary and features some returning characters. The story features a "massive flashback in the middle of it" and is in the genre of stories where "the Doctor doesn't appear till some way into the episode."
Episode 13 - Written by Neil Gaiman and directed by Stephen Wolfenden. The Cybermen will return and it will star Tamzin Outhwaite, Jason Watkins and Warwick Davis as a "band of misfits on a mysterious planet…". Started filming in November 2012. According to a leaked script, it's called The Last Cyberman.
Episode 14 - Written by Steven Moffat and is directed by Saul Metzstein. Features Vastra, Jenny and Strax. Some scenes are set in Victorian Scotland. According to Metzstein, features "genuinely scary monsters."
I don't know about you, but I can't wait for April