Beta 1.14
Update 4 has added a new enemy type, the Runner. The Runner will start spawning at round 3. The Runner is a shielded enemy type that darts across the screen, creating a segmented wall. The Runner and wall segments can be destroyed by your abilities and blast wave. Both will deflect all basic shots. The segments will be destroyed after 3 seconds on their own.
The Runner really mixes up gameplay by sectioning off parts of the arena for short periods of time, forcing you to adjust on the fly.