From The TimeSplitters Wiki

The Mapmaker has appeared in every Timesplitters game so far. It is one of the reasons that made the series so popular, and is very easy to use. Starting from Timesplitters 2, the Mapmaker has the added ability of making story levels. You can choose the Tileset, the music, the weapons available and the spawing points for the characters. In TimeSplitters Future Perfect, you can add a sky to your levels, decorations, baracades, and up to four Zeeps. Also, you can even create your own Assault game.

Downside

The more spaces you place, the more space you take up.

TimeSplitters 2

The small rooms cost alot of space compared to a Large Room. And spwaning points cost some space.

TimeSplitters FP

The downside is that you can only play up to eight players in each map.