Turns out that's more of a problem than I thought. There are, as it happens, multiple time portals scattered over each time period... it's not just the one. So I further defend where I've placed the flag as no portal brings you that close to it, as you have to go up to get the flag, but it does provide multiple ways to escape to the other side once the flag is grabbed.
Decided to do some exploring on this one:
EZkeel originally made this map for Unreal1 before converting to ut99, judging by screenshots he used. I was unaware of that room, or that there were 2 of them, so I decided to go ghost and check them out.
So, the rooms are 1 to each side and fairly identical. Haven't gone looking yet for the teleport that gets you in there, but getting out is pretty simple.
You goto that circular pad...
...and it pops you back on the map here:
I'm not sure what the significance of that room is. There's a translator pad in each room, but as hitting F2 in ut99 is fairly pointless (as it doesn't work for reading those) I'm assuming it had something to do with the U1 version.