Phalanxes of riot police, some set on fire
This is important: Most protesters are not violent, and what's happening in Kiev is not, despite some government assertions to the contrary, a "mass riot." Still, there's been violence, with some protesters throwing flares at the vast rows of security forces. This video shows what it looks like from the police's perspective, with some set alight at about 2:00 into the video.
Protesters built a giant catapult
Some protesters are taking the siege mentality in the square a bit literally and, on Sunday, built a big catapult with which to hurl small stones. According to RT, police later took it apart.
Riot police taunt protesters
This video, from the protesters' perspectives, shows some security forces taunting the crowds, including with an obscene gesture. Things are getting tense.
This is what the square looks like now
For all the drama of the three moments above, this video seems to better capture the status quo of Kiev's Independence Square. It's tense, with protesters beating sticks on the burnt-out ruins of cars and trucks and with riot police firing water cannons intermittently (brr), but it's not quite trench warfare.