Kashuan Keep is an interesting location. You can visit the first floor any time you want, and indeed the enemies you'll find there are relatively tame. Once you obtain the Goddess's Bell and make it to the upper floors, however, things get a lot tougher. As with the snow cavern, most of your fights will be rank 2, allowing you to more easily advance your skills. You'll also find some nice gear here, including several mythril weapons, a golden shield and suit of armor, and the powerful werebuster mace. There is no shortcut back to the first floor, so once you obtain Egil's Torch, use Teleport or Warp to return to the first floor and obtain the Sunfire.
Note that you will encounter Gordon here once you bring the Goddess's Bell, and he will join your party. A single MP increase will bring his pool to respectable levels, so you may want to give him a spell (perhaps using the Cure tome found on the first floor) and cast it a few times, then return to the Inn and Bafsk so you have some extra healing.
Also of note is that there is a chocobo forest directly south of the keep. You can ride wherever you like without fear of encounters. If you need to rest, consider returning to Poft to stay at the inn, then having Cid fly you back to Kashuan afterward. It will cost you 400 gil, but it saves a lot of time. Then again, Gordon could benefit from the easy encounters to build up his skills - the choice is yours.