First of all, can Poljen see the elf that appeared and attacked Cahleth up on the tower platform? If so, would his movement allow him to get there and attack this turn? I ask because by using a pike he should be able to leave his position without suffering an Attack of Opportunity. But if he can't get there AND attack, he most likely will stay where he is.
You're in a bad position to see up there. That stated, others by the barricades can see, and there's likely to be some shouting about it in a moment.
Unfortunately, you will have to double move to get up there in one turn. You could back away from the gate and throw/shoot something up there on this turn, but he'd have partial cover from your angle.
We can safely assume that Sarai would have decided not to use her oil flask, therefore staying by the side of the gate on the southern tower side.
So she's making her way back up there now, either slightly ahead or right behind Cratol.
Hopefully someone's got a rope or a ladder of some kind at hand, because the bard doesn't.
Nothing of the sort just lying around up there. There are ropes to be had around though, including the one used to rescue Poljen. If using the south tower for this, you will probably need to deal with the teleporting intruder before you can lower the rope in peace anyways, making any ropes owned by your companions up there available as well.
You could. Page 195 of the PHB for further details, but basically you can give up your attack to make a shove attempt. Your athletics check vs his athletics or acrobatics (His choice). Succeed, and you can knock him down or push him back 5 feet.
That's not usually a huge deal, but given the cramped space up here, pushing him back would probably cause him to topple over the low wall at the back and fall. You do have to beat his check to do that, but the odds are pretty decent, especially if you work together.
Edited: daishain on 5th Mar, 2017 - 10:13pm