I would say it means "dictates" or "mandates" in this case. Personally, I read your sentence as "because of the great strength of the fortress, and the people inside, we need to take the least amount of time possible crossing the channel. On a side note, channel is lower case. As a side note, in English context clues (words around the problem word) really help. In future, perhaps provide a few more words before of after the problem word (channel here) for a better idea. Hope that helps.

Hello Aloysha! In this instance, we can assume that the fortress is protecting or guarding (as that's what fortresses are for) the shortest channel crossing which would be the easiest and most logical place for anyone (including enemies) to cross. This means that the fortress controls who may cross the Channel and so it controls this crossing. So in this instance command means control. Please feel free to message me if you would like to ask any other questions.

The way I understand the context is that the fortress "controls" the channel crossing. Saying that the fortress "commands" the shortest channel crossing sounds like it has military control over it. Because the crossing is specifically noted as being the shortest, it sounds like a strategic advantage for the fortress.

I would take this to mean that the people, the military forces based in the fortress can control the crossing, so that nothing could make the crossing unless they allowed it.

Almost like it boasts that particular advantage. Hope that helps.